When someone's flying around with more loot than they know what to do with, there tends to be difficulty in organizing what's to be sold and what's to be kept. One way of remedying this is a means to mark inventory items with different symbols.
This may take the form of a small tab in the corner of an item's icon. By clicking this once, the tab might change to a star. By clicking again, it might change to a dollar-sign. This way, when sifting through your items at a shop, you have a quick reference of which ones you want to sell versus the ones you want to keep, as these symbols might appear next to the item's name.
Another handy feature could be that, if you attempt to sell a "favorited" item, a warning may be issued asking "Are you sure you want to sell this item?", while a button to "Auto-sell" at a shop will automatically sell items marked with a dollar-sign.