So, if a known scumbag posts an "I'll take it" in your sales thread you feel you would be legally obligated to sell them the item? What if the person who says I'll take it doesn't respond with payment? When are you legally unobligated to sell it to the person? How long do you legally have to wait? How do you know you are selling to someone who is legally allowed to own such a knife?
Further, the above bolded contradicts what Thomas said.
Yup. No contract until the seller accepts the offer to buy either by action or by the happening of the event the seller specifies as acceptance. ("I accept the first 'I'll take it" posted in this thread.)
Our society has taken years to work out the best rules for when a deal is made and what the terms of the deal are. As you point out, practical considerations may mean that breaking those rules has no consequence, but having no rules means no one knows for sure what's going on.
Example. You decide that you accept an offer to buy only by payment and the first payor gets the deal. But when does "payment" occur? The law has an answer, but that's "legal nonsense." So is "payment" when you physically get the money order, when you "cash" the money order, or when the money can no longer be pulled back by cancelling the money order?
And if I was the first to post say "I'll take it," and someone beats me to "payment" by using PayPal a day later instead of a money order, he gets the knife because he "paid first"? Sort of a race to "pay" with you as the arbiter at some point. (And when are you "paid" by PayPal? When the buyer can no longer file a case to get a refund on a phony "Goods not received" claim?)
Obviously, I think society's rules are the way to go. They still leave the seller in control if he is clear about the terms. Clearly, you think otherwise.
If the buyer does not pay, the legal nonsense says he has repudiated the contract and the seller has no obligation to deliver.
I don't know how you would know that a person is or is not "legally allowed" to purchase a given knife - or receive it across state lines (all those automatics that are illegally shipped from state to state and are subject to confiscation as contraband). I think a legally-binding representation that the offeror is "legally allowed" ("by offering to buy, you represent") should do. You should be protected if, down the road, the excrement hits the rotary device -- unless someone can prove you knew better. More of that legal stuff.