So I have this extra scrap in my balance and I wanted to get it out by buying a scrap. I can't, and this is what the trade tells me:

http://i.imgur.com/NJVEcmK.png

Why does it do that?

"0.11" is not always equal to a scrap as a scrap is actually the value of 10 over 9, which is a loop and cannot be represented in 2-digit decimal representation. For pawn shop price "0.11" is the value of 10 over 9, but not an exact number of 0.11. If you have 1.00 and purchase 0.11x9, you end up with 0 but not 0.01. This is the basic of credit calculation on this site.
Simply put, you may earn or lose a little when trading with pawn shops depending on the price of items (when the owner puts up price like 0.12, you are spending a little bit over a scrap while it still looks like "a scrap" to you).