An alternative to locking the threads could be to ensure that there is a photo and at least a vague description (including final price if possible) which is saved locally to the thread to allow the sale to be used for market research for longer than 3 months (or the day after in the case of gumtree ad's). Having a bunch of threads titled "check this out" and containing nothing but a link and some vague comments is about as useful as the threads titled "what's it worth?" with no pictures or any real idea on the condition of the vehicle (can you tell I hate "what's it worth" threads).

Might make it easier for people to determine a price on their own without being redundantly repetitive.
I'd hate to see a situation where Byron looks at a thread where he has answered the same question a thousand times and just say's to himself "f*&$ it, I'm not a parrot" and gives up.

Need yourself a "donate" button somewhere too. Surely you're not paying for this whole site on your own.