Sometimes, we want to carry around items in the cargo hold that we're not using at the moment and that we don't intend to sell immediately (or ever). Examples might include:
-quest items
-ID cards
-enhancers (waiting for the right item)
-gems/spheres
-high level ores
-etc.
-analyzers with more than 0 charges

It would be helpful to be able to mark such items as "not for sale," preventing them from appearing in the cargo list when attempting to sell items at a station. This would both avoid the risk of selling the item by mistake and also shorten the list of items that would appear, making selling more convenient. An alternative implementation could be that such items appear at the very bottom of the cargo list (or at the bottom of the "sellable items" list if we're at a station that is only interested in some types of items) and require a second confirmation before actually being sold.