Personally, I like this idea. Perhaps you could only sellback for full price if you haven't left the shop screen yet? Certainly, combined with the minion exploit, this could lead to bad things. What I don't see is why many of you are deciding to be so hostile (and, dare I say, elitist) towards an idea which won't hurt game balance and will help people who are learning the game have an easier time.
Occaisionally, just occaisionally, you'll make a mis-click. Why should you have to lose hundreds of gold (minutes of travel and farming) for that? Just so you can be "punished"? Sometimes, this community bewilders me. Actually, make that all the time.
You learn your lesson in a hurry after buying the wrong item. Lets just coddle people, i'm sorry you got the wrong item timmy, there there, suck at my teet, it'll be alright. The possible ramifications of being able to sell stuff back at full price FAR FAR outweigh any good it could possibly bring as far as idiots buying the wrong items.
Is the half a second it takes to double check you're buying what you want doing to make or break the game? Not a chance. Calm down and make sure you're getting the right item. Doing "ZOMG!!!" *panic, freak out clicking* at the shop isn't going to benefit you, or your team in any way.