Uhm... now it sounds complicated. On one hand, those who want to play a game with regular "magic items market" like in some previous editions, need a price tag on each item. On the other hand, for those who don't want that sort of game, a price tag is detrimental because it inevitably creates the assumption that there must be a market as a default in 5e.
Furthermore, price tags don't work in a vacuum, their meaning depends on treasure, so writing 1000gp besides a +1 sword means nothing until someone has decided how long it typically takes a PC to gather 1000gp.
So to actually have a default magic items market for the first group of players, I think the game needs to provide not just price tags, but also treasure encounter tables and wealth-by-level guidelines (for starting at higher level). Wealth-by-level might be enough by itself, and the DM can just spread the total treasure over different encounters without strict rules about treasure per encounter.
It sounds to me too, that this should be clearly labelled as OPTIONAL.
I think most people just assume price tags will be added to each and every item, which once again will easily create the feeling that every magic item can be bought and sold, but there is another way. If every magic item just had a "magic item level" in a way similar to how monsters have a CR, then this "level" could be used as a gauge and be tied to different OPTIONAL meaning in a group's game.
For example, for items that replicate spells, their "magic item level" would be often just the minimum spellcaster level at which the PC normally get access to those spells. This would create a basis for estimating the "level" of other magic items. Then it would be easy for a gaming group to play so that normally they won't expect to find items of levels higher than their current characters level, but another gaming group who wanted to play a lower (or higher) magic campaign, could just go with "level - N" (or "level + N"). This would be the basic feature of a very simple "magic item level" system, and it would be the only thing to add to each item's description.
In addition, the DMG could have separate OPTIONAL rules to:
1) assign price tags based on magic item level, maybe some simple formula depending also on how many properties the item has (so that an item granting 2 spells costs more than another granting 1 spell of the same level)
2) using magic item level as a character level requirement for the creation of such item
3) using magic item level as a character level requirement to actually use such item, if the gaming group wants a WoW-like approach to using magic items
4) using magic item level to determine the properties of an intelligent version of certain magic items, so that they work a little bit like NPCs
Once again, each of these would be optional and completely independent on the others, but each item would only need to have a "level", everything else is taken care by these additional rules and doesn't need to be written in each item's description.
