I usually host any pics of anything I'm selling on my own site and delete once it's sold, simply because it's not my equipment any more, why would I host pics of someone else's equipment?
I'm not sure I really agree with an archive of historical sales, surely any sale is the business only of the seller and buyer? If you plan on buying anything, surely you save any pics and information before commencing with the purchase anyway? Ebay, for example, lists items only brielfy once a sale has completed, I can't think of any other site which would store information publicly on users purchases which could quite easily be used for sinister purposes. Let's be honest, we all put up a lot of information on our equipment and when we'll be out of our houses at gigs, for example. Do we need a searchable database of equipment purchases and the amount of monies spent on equipment going back over years available to absolutely anyone?