I'm all for ways to buy/trade/sell off magic items, but I also want to stay in tune with the 5e theme of no MagicMarts.
What I decided to do was apply a little bit of economics to it.
Magic Shop - A huge city (like Waterdeep) may have a few shops where one can buy magical items. They're the high end shops of master crafters and merchants so cunning that the Thieves' Guild doesn't so much as pass through his door. The alchemist will have healing potions, but may also have a few other potions/salves of this and that at any given time. But while you might find a Flaming +1 sword (as an example) for sale, you won't be able to walk in and order a +1 sword with Keen and Icy Burst. You have to buy/trade from their selection on hand.
(In this, I add in the sorts of weapons the characters want to use. Say that random treasure gave a +1 Flail but the player likes Warhammers, letting them trade for the role play preferred equivalent isn't game breaking at all to me.)
Research - Hitting up a huge library (like Candlekeep) during your downtime and being willing to shell out some gold finding what you're looking for is a good way to get things going. The more you're willing to spend researching, the more likely you are to be led to the necessary item. Note, that researching an item in this manner only gives one an accurate locale. That location may very well be "within a Yuan-ti egg chamber".
Common [A few hours]: Very easy. Around 25g or so.
Uncommon [1-3 days]: 25-50g (50%), 50-75g (75%), 80g+ (100%)
Rare [1-5 days]: 100g (25%), 200g (50%), 300g (75%), 400g+ (100%)
Very Rare [3-10 days]: 500g (25%), 1000g (50%)*
Legendary [14 days, minimum]: 2000g (25%)*
*You can't generally get any higher than this without some really potent aid. A master archaeologist, a wizened scholar specializing in this kind of thing, etc.
I was going to add an Epic Level Consortium or Enigmatic Peddler of Exotic Wares, but I figure the DMG will be out before getting to that point, so it won't be necessary.