The DMG. Page 135: "Unless you decide your campaign works otherwise, most magic items are so rare that they aren't available for purchase."
In 3e, the default is "If you're in a big enough settlement, you can buy magic items costing up to X gp (depending on the size of the settlement)." There are also hard and fast rules for making the items, and it's not that difficult. 4e is even more friendly toward item commerce, where roving merchants sell them even in rural locations, and making them is even easier (though not as cost-effective).
But in 5e, the default is "You can't buy or craft magic items other than potions of healing." There are some guidelines in case you do want to allow characters to craft items (and doing so is a lot harder than 3e and 4e), but you're pretty much on your own if you want to have them for sale.