Thanks for the earlier clarification Phalynx.
I have a small feature request, if possible: what do you think of a "discard" option for items, both purchased and received as a gift? That way, people can get rid of the things they buy even if the "Sell item back to shop" option is disabled. This would also provide a way for users to discard items that were gifted to them. Currently, they are stuck with the items they've been given, which can be a nuisance.
On a slightly related note, what do you think about allowing us to enable the "Sell item back to shop" option per shop item? Maybe have a checkbox next to each item listing in the "Edit Items" view which would enable or disable the "Sell item back to shop" option per each item. Along with this should be a checkbox at the very top of the column which would enable or disable all items in a category, making it easy to switch on and off entire categories.