I think it's difficult to prevent... For me, I think the mods and the rest of us cannot be NANNY to everyone. Most of us buy/sell things based on Price, Reputation as well as personal experience. Like for instance, most of us will not buy expensive stuffs without inspecting it.
EVen if we have all the pre-caution in place, you may still buy "lemons" or you may be plain unlucky.
A lot of these sellers, they are probably those who doesn't post but may be surfing SRC for years. When it's time to decom, i don't see why they should be penalised just because of their inactivity. Just let them sell, end of the day, the price, condition of the equipment will determine if it's good deal.