I'm just doing some brainstorming here but wouldn't it be neat if an ad in the classifieds had a limited life-span. And when the time runs out for the ad the seller gets bumped with a query if the item is sold. At this point the seller gets two options:
If the item is not sold he/she get the option to extend the ad for another period.
If it is sold the seller will be asked to fill out "a form" with the price the item was sold for as well for what tool it is. Maybe just use scroll-down boxes with multiple price-range options as well for tool-model. After that the ad will be removed. The collected data will be stored in a database like a poll where the average price, number of items sold and time-span eg:
ETS-150, $300, 2012-2014, 4 items sold
RO-90, $400, 2013-2014, 2 items sold
This way a potential seller/buyer knows how much would be reasonable to ask/pay for a used item.
If this is executable i don't know, just thinking out loud.