I think one thing to keep in mind with this is the old saw about buying goods . . . 'You can buy it cheap, in good condition, or fast- pick two.' The inverse values apply to selling items . . . when you try to buy something fast, you are going to have to throw the cash around to get it quick- just think of Amazon and how much more something would cost if you want it the next day.
So when you are trying to sell something, the biggest difference IMO is that if you have to sell it quick you are not waiting for the best offer or trying to invite a bidding war between buyers. Instead you are likely finding a 'wholesale' buyer who is going to give you at best 60-80% of the value depending on what the item is . . . and that is because they have the capital/overhead to hold on to the mech or weapon system until a buyer comes along to pay out what the market value might be- or until they get someone who is in a rush.
Tommy's Trouble managed to salvage a Panther 8R from the pirates that hit their garrison post and recovered two slightly used heavy APCs. The two months the mercs remained on the planet they put the Panther back into operational status. They arrived on Galatea and went into their R&R cycle, putting up a notice on MercNet that a used Panther was for sale. Two weeks into their R&R cycle, Tommy got a rush contract offer from MajestyMetals that would require them to leave immediately for the periphery.
Tommy's mercs have a retainer with Dewey, Cheatem & Howe for legal documents related to contracts but they have no other representatives or personnel that will stay on Galatea. Tommy does not want to pay the cargo & storage fees for the Panther being shipped with them- besides, the Scrapheap has a rebuilt gyro that would work to replace the faulty gyro on Clint's Marauder but they do not have the cash on hand to buy the gyro. The Scrapheap's owner is not taking anything in trade for the gyro except cold hard c-bills. Unfortunately, no one has wanted to pay what Tommy was asking for the Panther on his MercNet ad . . . but some purchasing agent for Rod's Used Mech Market had sent in a real lowball offer at 60% of the value. Which just happens to pay for the gyro with a little more for the warchest. Well, it is not as much as they want, but getting that new gyro will keep the Marauder from falling over as often or not being able to run . . . besides, if they can finally figure out the gyro's problem they can sell that in the future to recoup some of the costs. So Tommy sells the Panther to Rod's Used Mech Market at 60% of the value so he can go buy the only gyro on planet for sale that works in the Trouble's Marauder in his price range, knowing it is probably 110% to 120% of the market value.
For the mercenary trade, the mercs as the end user sell surplus equipment or salvage for less than market value but will be buying at over market value (what a House would buy from a factory) due to demand & limited supply. The people and companies doing the refurbishment/repair work are the ones pocketing the difference, much of which goes to paying their expenses- like the parts, labor, tools, and facilities where the equipment is repair & stored until sold. IMO mercs stand to make the most from the value of what they sell if they are selling directly to other mercs- or to some planetary noble or corporate security buyers as long as the mercs are NOT in their jurisdiction.
In the above example, if Terry's Trouble had stayed on Galatea longer, they might have been there when the Random Ronin had arrived on world with a ComStar cheque covering full re-imbursement for a used Panther 8R lost when a Lyran unit raided Acme Ammo Plant #13. The Panther's pilot survived the loss of his family machine, and is looking to replace it with the same type mech. While that pilot might not have been willing to pay the 105% of the value the Trouble might have been asking, they probably would have sold it for 90-95% of the value which would have been more than a middle-man would have bought the mech for when they had to leave the planet.
Instead the Ronin pilot had to go to Galatea City Savings & Loan to get the extra 10% (and Lyran taxes?) of the value Rod's Used Mech Market (did Rod's get a kickback from GCS&L for referring the pilot?) is asking for the Panther because the Terry's Troubles left 2 weeks before the Random Ronin arrived. Neither merc command paid a retainer to have a business administrator able to handle either type of transaction (who would get their slice, think consignment/realtor slices) to save them a bit of time and money. Thus do the middlemen survive and make money off the merc business.
As for other two I mention- cheap or in good condition? Well remember you can buy and sell 'salvage' grade equipment besides the complete/functional/operational equipment. Sometimes buying stuff broken is a good deal- you have good techs, they have time en route, or you are going into a garrison contract so your mercs have time to repair whatever you bought. Buying it cheap means you spend sweat/favors to save c-bills . . . this is what having merc commanders, lead techs, or administrators with positive contacts can do for you. Your old regiment buddy is assigned to the garrison on Galatea and he heard Galatea City Savings & Loan seized the remaining equipment of a failed merc command that managed to get back to the merc world but defaulted on the loan. The seized equipment is not for sale yet, but rumor has it one of the seized mechs was a Panther though it might not have a functional PPC. You might be able to get over there to make a offer before they go to auction . . . GCS&L saves on auction fees and frees up space in their impound lot and thus lowers the price some.
The other thing affecting price of equipment is another common business refrain . . . 'location, location, location!' Simply put, buying or selling prices are often tied to the location you are carrying out the action. The old Combat Operations sourcebook had modifiers for equipment and munitions bought across borders . . . FREX, it had a modifier to the then BMR (now TM) price of LRM Thunder ammo if you buy it outside the Capellan Confederation . . . or AP/Precision AC ammo when outside the FedSuns . . . or C3 components outside Kurita space- though the mark up was not as severe b/c the tech had already been in proliferation a bit. I think FM Mercs(R) also had some modifiers to buying equipment based on the merc hiring hall you were at when trying to make the buy roll. I KNOW the rolls got harder (and thus more expensive) the more specific you got . . . roll for a mech? roll for a heavy mech? rolling for a Warhammer? rolling for a Warhammer 8D? Then you might be told something was not available that 'cycle' because your odds got more difficult.
Another aspect of location is . . . are you in someone else's jurisdiction? If you have the contract to garrison Acme Ammuniton Plant #13 and salvage a Warhammer 6K from when some Dracs raided the world, Graf von Snuffy might make you a low-ball offer during dinner when you are visiting their estate on invitation. You do not want to sell Commander? Ah, then you are welcome to be my guest for a little while.
While the above is a abuse of power, location still comes into play with supply and demand . . . you need parts? If you are garrisoned in the boonies where a cargo DS stops by once a month it is going to cost importing it- both in shipping and trying to find it since the seller is likely going to be someone who was sitting on it to get the best price. Galatea or Outreach? Not going to pay transport and likely not going to find someone trying to gouge you either.
