First firm "I'll take it" should get it. This is a simple, unambiguous practice, available to everyone. A seller shouldn't be expected to wait around while someone examines pictures, then decides. Then the question becomes how long he should wait. One day? Two days, a week? All these issues go away by honoring a simple I'll take it.
There is another issue with "I'll take it's", however. This has happened to me twice. I fire off an E-mail saying i'll take it, and then post an I'll take it on the thread. Simultaneously, someone first posts I'll take it on the thread, then sends the E-mail.
In both cases, the seller honored the E-mail and I got the knife. If he had decided to do the opposite, I would have understood. If I ever post a knife for sale, I will clearly state that the first E-mail take it will rule. If a seller posts his conditions, then no one has any grounds for complaint.