I am not a fan of the auto delete idea, as sometimes it is nice to pull up an item that was available in the past in search and find it is still there. Some members are very popular, their pm's can fill quickly. Having the response links can generate interest, as this is a live forum and can help you find other avenues.
Instead breaking the forum into three areas like buy, sell, and trade maybe split it up like this:
chicken hatching eggs
other hatching eggs
Live animals
supplies
other items (perfect for all of the great items our members often offer that make themselves, especially at Christmas time)
Psst... many people are impulse buyers and don't usually search for a specific item. Would it also be possible to lengthen the page on the buy/sell forum. I presently already use the feature that when I click on a topic, it expands to show the first 75 messages... I would love if just the buy, sell board could do the same so when I am off board for a while and click on, I don't have to click onto as many pages.
Then have your stickys cross all of those boards to tie them together...