sell by auction or not?

[C.O.B.]

I would imagine there would have to be some form of a cost basis that the seller would have to prove for there to be a gain. If I sold my collection of Pre 64 M70's for $X who is to say there is a gain or not?

[G. Wells]

I think you might have it a little backwards about having to prove a cost basis to establish gain. Worst case, if you were audited by the IRS, you might have to prove basis in order for it all not to be treated as gain. See the prior post about signing false tax returns.