I think the general rules of the board should apply at all times, which include no advertising. Swap/sell could work, but you would still have to refer to a legal escrow/payment website since we cannot take the burden of handling sensitive transactions like money deposits by ourselves. Besides that, this is a public forum, anyone signing up could simply browse to the threads and find someone else's paid-for script ready to be copy-pasted (unless of course, scripts are exchanged through email or PM or something, but that would pretty much defeat the purpose of PHPHelp I'm afraid). Just a few thoughts running through my head.
I agree with the no advertising (as far as companies coming in and posting direct advertising).
And my thoughts are just a "Friendly" Post that states something like. "I have an XYZ Computer with A, B, and C in/on it. I would like $500.00 for it. If interested let me know."
From there you could deal with it in PM, email or even in the forum. When it's done... Lock the thread.
This forum would NOT be responsible for anything other than being a meeting point. The arrangements would be made between both the parties. I guess I just kind of see it as a "Classifieds" for our community.
As far as payment goes, I would certainly recommend Paypal, escrow, or something similar, but again that would be up to the parties.
Were we to proceed with this idea, I would certainly have a disclaimer that would need to be acknowledged prior to being allowed to access the area.
Maybe instead of something like that in a forum, however, It could be done in a different area and just referred to from the site. I don't know.. I guess it's just my ramblings of things to do.