Some other comments:
- many buyers are just browsing RFF, and many are not even registered. Just because they may respond first, does that make the seller obligated to sell to them? Personally, I'd rather sell to someone I know and can trust. Otherwise, I'd just put it up on ebay.
- buyers frequently change their minds. Until the payment is made, the sale is not final. Should the seller not be allowed similar latitude?
- I hate to say it, but some buyers are just a pain in the a**. Nitpicking my equipment to try to lower the price, impatiently emailing and PM'ing like the seller has no dayjob. Personally, I would like to be able to decide who to sell my precious equipment to.
- Posting a reply to the classified thread is no guarantee you were first. I think that's a no-brainer. In fact it even dissuades others from replying, thinking the item is no longer available, when the poster was not really committed to buying the item.
- I think responding to an item should be differentiated from commiting to buy it. Some PM their interest, then proceed to ask a series of questions. I don't think that makes for a commitment, as any "negative" answer to any of those questions would make him back out. If you are not commited to pay until those are answered, you surely do not expect the seller to be committed to you either.
- Also, especially with regards to international buyers, items like mode of payment and shipment need to be worked out. I personally do not want to give out my bank account info for bank transfers. Until all those things are worked out, nothing is final.
Bottom line is, for me at least, RFF is a community. The classifieds, not purely business. If I wanted a quick sale to a stranger for (likely) the best price, then I would sell elsewhere.