Again, unless the characters decide to spend a significant amount of resources establishing the contacts/reputation/time, then they can't get the full value. But if you want better reasons, why do you pay more money to buy something from a store than from ebay/craigslist/garage sales? Because you have reasonable trust in the store brand, the return policies, established levels of quality, etc. Also, stores are regulated as a business by different agencies (BBB, consumer protection laws, etc). For your characters to become a business, they need to establish themselves as a reputable provider of magic items. That starts at selling their items basically at the wholesale price (1/5 market price). After a few years and many many sales, they can have built up a name for selling quality magic items and can then charge full price, assuming they've spent those few years just selling magic items. So I really don't think its reasonable for characters to say that they should sell magic items at full price unless they're wanting to play a merchant campaign.