Hey Richard, i gotta chime in as well. Its been a couple years since i took basic federal contract law, but as I recall, I think you and Rick are both partially right. If a seller comes up to YOU, and specifically you, and offers you something, then that becomes an offer (assuming all other the requirements of an offer are met). If you accept, then its a done deal. So If i were to say "Richard, I want to sell you my 30 tarptents for $10" and you say "awesome!!!, I'll take it"... then If i dont fulfill my end of the deal, you have a claim against me.
However, I believe what goes on in these gear swap forums, is actually called an "invitation to treat" The seller isn't directing his goods to anyone in specific, so its not really an offer persay. When you tell him that you want it, you, the buyer, becomes the offeror, and the seller can accept or not. From Wikipedia, under Invitation to Treat
"A shop owner displaying their goods for sale is generally making an invitation to treat (Pharmaceutical Society of Great Britain v Boots Cash Chemists (Southern) Ltd  1 QB 401). They are not obliged to sell the goods to anyone who is willing to pay for them, even if additional signage such as "special offer" accompanies the display of the goods. (But see bait and switch.) This distinction was legally relevant in Fisher v Bell  1 QB 394, where it was held that displaying a flicknife for sale in a shop did not contravene legislation which prohibited offering for sale such a weapon. The distinction also means that if a shop mistakenly displays an item for sale at a very low price it is not obliged to sell it for that amount ." Generally, advertisements are invitations to treat, so the person advertising is not compelled to sell to every customer. In Partridge v Crittenden  1 WLR 1204, it was held that where the appellant advertised to sell wild birds, was not offering to sell them. Lord Parker CJ commented that it did not make "business sense" for advertisements to be offers, as the person making the advertisement may find himself in a situation where he would be contractually obliged to sell more goods than he actually owned."
I am not an attorney, but it appears to me that sellers on this forum are more like the store owners mentioned in the case law. Either way, I hope no one is arguing contract law, and small claims when it comes down to gear