Since it seems to cause a lot of confusion, I will be very specific.
Do not link to or talk about an active ebay auction. Linking to ones that are finished is okay. Talking about Ebay, or suggesting that someone search there if they are looking for something is okay.
Just to make sure that I'm being perfectly clear, the following is okay:
Ebayebayebayebayebayebayebayebayebayebayebayebayebayebayebaythereisstuffonebayebayebayebayebayebayebayebay!