You should NEVER give your players items they will just sell.
A lot of the love for random loot comes from 3.5, where that was how you were supposed to give out loot. It's important to remember that 4e is not balanced around random loot; instead, it is balanced around the idea that every single item the party finds will be useful to them. Gems, art pieces and other things that exist only to be sold are fine to give a player just to sell, but the equipment you give them needs to be stuff they'll keep and use.
Consider the differences in how awarding loot is handled in 3.5 vs 4e. In 3.5, you have the Wealth by Level chart, which tells you how much stuff the party should have. If they don't have enough, the DM just keeps giving them more until they have the right amount. No matter what you give them, it's easy enough to look at their character sheets & see how much they have, and adjust to keep them at the proper point.
In 4e, by contrast, you use the parcel system, which rather than telling how much the PCs should have tells how much they should get. Consider two parties, A and B. Every item A gets is off their wishlist, so they only sell gear when it's been obsoleted by a piece with a higher enhancement bonus. Every item B gets is useless junk: weapons & implements & armor that nobody in the party can use, consumables that give situational bonuses for situations that never arise, weird useless wondrous items, etc. Because their DMs both follow the parcel system, parties A & B both get the same amount of stuff. But because they're selling everything to buy gear that's actually useful to them, party B has 80% less stuff.
The parcel system is balanced around the idea that the party is getting items they want (or at least will use) rather than vendor trash. If you're following the parcel system and the players insta-sell an item they found, you need to give them a replacement item. If you don't want to do that, then you should abandon the parcel system entirely and work out a wealth by level chart to use. To make a WBL chart, I would recommend adding up all the parcels the party should have received up to that point, reduce the value of certain older items by 80% to simulate selling off obsoleted enhancement gear (3 per PC at level minus 5, level-10, level-15, etc.), then divide by the number of PCs in the party. That's about the amount of wealth each PC should have. Remember that gear they have is valued at what it would cost them to buy it.
TLDR: The parcel system is balanced around the idea that players use everything they find. If they sell something without using it, they're only getting 20% of what they were supposed to get.