"i'll take it" is an offer to buy. You accept that offer by saying you accept, by sending the knife, or by saying "I'll take it means the poster of those words has purchased the knife." Otherwise, there is no deal and you are free to sell to someone else. I think the custom here is that "I'll take it" obliges the offeree to promptly pay. Promptly. That includes promptly doing what needs to be done to arrange payment - as in asking "What's your PayPal Account Number?" if he it's not in the sales thread. If there is a deal, delaying payment is low class, even for non-"professional" buyers.