Simple: if you can't afford the thing at that very moment, but will have the plat for it in X amount of time... you can either be completely locked down in your listsold, or you can reject it. What if they're not online, so you can't explain why you can't buy it then? You reject it, they sell it to someone else. What if you sent yourself [item y] from an alt to sell to have enough plat to pay for the item? You can't get to it without rejecting the item, and again, if they're not on, you might just be fuckered.
What happens if you make a post on the forums saying you'll buy Easy tmaps for X amount, Mod tmaps for Y amount (the same amounts you auction for a few times a day in-game), hoping that someone might send you one every now and then, and that day when you get home from work, you find that you've got mail waiting. You look, and think, sweet! An E map! And you accept it. And then another. And then another. And then another. And then you see you've got a mod map... and another, and another. And then two more E maps. And another mod map waiting! You don't have any way of knowing whether there are more in line, and you may be running out of money. Personally, I think it's more than fair to put priority on, say, a set of augments you specifically told someone you'd buy and which were made for you specifically, over tmaps sent at a standard price without any communication. You'd want both, and you'd pay for both, but if you had to spend a few hours, or even a day, farming, you'd want to meet your specific obligation first.
If it's difficult to code, that's fine, I understand. But the feature IS beneficial for anyone who buys stuff frequently via listsend, and I think that the above-mentioned disadvantages are by and large phantoms.
