Having sold, traded and given away a thing or two here, I'll take a shot at this one. The downside of going with the first " I'll take it " is, that there are a few folks that:
A: Get excited by an offer and forget that this is not the home version of Jeopardy and that ringing in early without regard for expeditious payment or the terms of the sale, doesn't entitle them to the item.
B: In the time it takes the seller to figure out that the first " I'll take it " hasn't got their act together, they may well have lost the interest of other, serious buyers.
C: Would be " I'll take it's " need to remember that it's not their property until it's paid for. The seller sets the terms of the sale in their listing. Which can easily mean, the seller may choose to go with a buyer with whom they have a positive track history or anyone else they choose, for nearly any reason they choose.
Chiming in with an " I'll take it " and then sending the seller a p.m. stating that they are not in position to pay for it, or any note that runs contrary to the terms of sale, nullifies the would be buyers request.
While communication is the key to the deal. Sellers need to remember that the communication on every listing starts with how well the listing, including the terms of sale are written.
Now if someone adds to their listing," the first I'll take it, gets it ". Well then buddy ole boy, I can't help you there, you're on your own.
Dave