I would expect to find a lot of wildly assorted 'Mech pieces in a Solaris VII junkyard, along with a few mostly intact but unrepairable 'Mechs (either due to scarcity of parts or problems which proved unfixable), because a lot of 'Mechs will get cored or irreparably damaged, and the high degree of customization and non-standard designs for arena combat would result in a lot of "unique" pieces that aren't all that useful to anyone else. "Here's a left leg for a Flashman....good luck finding a Flashman that needs it". "Don't have a clue what this is from, must be something custom built". "Gee, all this Stinger needs is the entire torso, and a broom to collect the pieces".
As for 'Mechs, I'd expect to see a high proportion of Locust/Wasp/Stinger pieces or other light 'Mechs, due to their relatively high production and relatively low cost. A lot of aspiring Arena Champion wannabes will have managed to scrounge up enough for one of these or some kind of industrial conversion, but not enough for anything decent, and there are several smaller arenas which feature the lighter classes.
There would likely be a much higher proportion of scrapped or badly damaged vehicles in a mercenary hiring location, and a bit more in the way of "common" 'Mech parts, but mostly in really bad condition. The better stuff would get picked through regularly, with the real junk tending to collect over time. Here's where you'll see the more combat-oriented stuff, particularly the mass produced units that mercenaries will have the most access to and interest in (for parts availability), although the better maintained stuff will tend to go quickly, leaving the "junkers" with more permanent problems. Minor issues like bad actuators or a dysfunctional secondary weapon will tend to get repaired (the repair will increase the value of the unit substantially, at a relatively minor cost), while things like flaky gyros and intermittent targeting system glitches will be harder or more expensive to find and track down or replace, so some mercs will unload them after failing to repair them. Some sellers may have gone to great lengths to hide the problems.
A skilled technical crew could probably get some of the poorer stuff functional, but a lot of it would be things that other techs had previously rejected as unrepairable, or at least not worth the time to fix and the expense for replacement parts: "Ah, here's a Martel Medium Laser....but the whole focusing array is sitting crooked, which means the framework is bent, and some of the electronics look like they've been burned.....I'm not sure if it can be fixed for less than the cost of a new laser." It might still provide parts to fix another unit, if you already know what's wrong with yours. It also might supply parts which turn out to have other problems, which you discover after you've bought the salvaged equipment and cannibalized it to fix your own.
The dealer's "prime real estate" would have the better stuff, possibly including a couple of complete and fully functional 'Mechs or vehicles, and "rebuilt" weapons in better condition. The junk would tend to gravitate toward the back of the lot.
This is a GM's equivalent of a "target rich environment". Better warm up those dice.
