What the actually
DMG suggests is that whenever a player needs to bye a particular item, a "mysterious stranger" appears with that item for sale. In other words, if you got the cash, the mysterious forces of the DM has the wares. How to express this in each campaign is up to the DM. However, there is a 30% to 70% markup. If you enchant your own stuff, you avoid the markup, but cannot make items over your own level.
Note that the rule saying you only get 20% of an items worth if yous ell/disenchant it almost guarantees you'll be cash-starved, so don't expect to be buying/making many items in the points-of-light economy.