If you play in a game where your DM is a stickler for inane rules like the 20% selling price rule, then you might be better off playing with another DM. Ours, makes it somewhat tricky / difficult to get the specialty items you want (this was even pre-Essentials rarity rules, it just made sense to him as a DM and we were all fine with it), but since we befriended some high level wizards and nobles, we can access a better re-sale value (especially as a reward for completing quests for said NPCs), and thus you could see the difference between 20-100% resale value as another, hidden level of gold reward for successfully completing a mission. It's all about who you know and how you talk to them.
Once, our rogues even managed to convince the thieves' guild PO to buy a crate of poison at a markup from the PHB price, by simply bluffing them. Of course, this is risky because a failure could mean immediate negative repertcussions, or at least expulsion or ostracization from that guild. It was nail-biting and fun at the same time, which to me is the point of Dnd. The players shouldn't get stuff handed to them on a silver plate, but neither should magic items disappear from the market for such items like a cash sink in an MMO bazaar. Those items should still exist and be locatable, by an enterprising thief who wants to pull the coin-slot trick by selling a rare item then stealing it back again. Lots of fun adventures can be had this way, by merely letting players do what is possible to do, and let the dice and common sense decide what's do-able.
In theory, you should be able to buy something that you could just steal, except at a huge price. The DM can always make the selling price of an item your party gold +1gp to keep it out of your hands. Why not steal a whole armory worth of stuff? Isn't that what rogues are supposed to do? You might have to lay low or try to fence it all in another kingdom or have assassins trailing you for many levels, but that's part of the fun. I really, really hate all this nonsense about not being able to buy items. Any given particular rare item might be hard to find / buy, but rare items cannot that different in price if their actual mechanical value isn't that much better. For instance, why would a merchant buy a lackluster rare item that he can't sell at that big a markup, when a common item gives him a 400% markup, as others have noticed?
Uncommon vs common is a different issue, and I really hope to not be spending endless hours arguing with these new rules that with access to the royal high court wizards and armory that it would be that difficult to trade, say, a Staff of the Storms +2 (worth 5000gp, and uncommon), for some Iron Armbands of Power. Upgrading the tier of these items should also be do-able as an exception. Like just pumping up the +1 to a +2. If a player can do a +1 to +2 upgrade with Enchant Magic item on a common item, he should be able to do it on an uncommon or rare too. Why not...the property hasn't changed, just the basic bonus.
That's an incredibly annoying nerf they just did for Enchant Magic item, right when we just got a ritual casting wizard join our group.