I'm a bit unsure as to how a regular member/subscriber can lock a thread in classifieds, so I'll let Sean or someone else field that one. If you have an "administrative" tab in the top right corner menu above the first post in the thread, there should be a "close thread" option there. I think that's how it should work, but logged in as a moderator, I'm not positive.
We have no problem with potential buyers making offers on "best offer" sales, as long as it doesn't become disruptive. Some have even proposed auction style sales (not necessarily with eBay rules--for instance the seller could decide to take the last bid that remains unchallenged for 24 hours, or with other rules), and the problems that people predicted have just never come about. The truth is, if people want an auction, they prefer the established rules and systems of an auction site, and we can't provide that. If the terms of sale are too onerous, no one will buy. If no one wants to do the research necessary to make a fair offer--which I always do whether the seller names a price or not--then sellers will figure out that this strategy is unsuccessful. The market seems to be regulating itself fairly well, so as long as it doesn't impinge on the culture of the forum, we'd prefer fewer rules to more.
The donation has always been voluntary. It's up to Sean to decide if that should change.
One thing that's come up that we haven't made a rule about is "bumping" threads. Personally I find this a bit silly, and it's been mentioned by a number of users as a source of irritation. For now, we'll just recommend that if you want your ad to have more visibility, or to be more successful, then provide more information or new information, if you're going to comment on your own ad--try better photos, examples of what the lens or camera can produce, a demo video, offer extra incentives like free shipping or accessories, or try a lower price.