Allow them to be "moved" to an archive or whatever but allowing dealers to delete their threads as soon as they consider them "sold" is not a good idea.

In my current instance the seller has deleted the thread I bought off, I left them positive feedback once it arrived and it looked OK, but am noticing quite a few things 'off' with it (and I'm not talking a crown guard 0.01mm too high or a shade of gold not 100% gen-accurate ) but I can't go back to check if I'm imagining what they claimed in the advertisement because they've deleted the thread.

Perhaps only permit them to move to an "archived sales" thread? I get there may be reasons they don't want their entire sale history sitting there in thread, from basic clutter to the simple right to manage their own business exposure, but it's not without big flaws... perhaps at least require a sale thread to be up for say 90 days before it can be deleted?