Because the seller is the one who sets the terms of the sale, he has the burden to 1) state them clearly, and 2) abide by them.
If he says, "First one to post 'I'll take it' gets the knife," that could be understood to mean the first person who posts "I'll take it" in their thread. The seller would do better to be clear on his terms, such as "First 'I'll take it' I receive in this thread, or by PM (but not by email) get it" or "First 'I'll take it' in this thread gets it" or whatever. Even, "Send me your 'I'll take it' posts here, by email, PM, or carrier pigeon, and I will arbitrarily decide next week who gets the knife" would be OK, as long as the terms are laid out up front.
It is ALWAYS better to lay things out up front than try to straighten out problems later.