You may be buying the wrong item. Make sure you're using the most recent version of the mod; I've never run into that problem, but if it persists I can look into it.
To select an item in the most recent version, you must press space on the item you want to buy. That item and its price are then displayed on the top-left, and you can then buy it with E. Sorry if it's a bit confusing. I'll see if I can find a way to simplify it.
