Changed Magic Items (houserule)
Pearl of Power. Can take expended higher-level slots and convert them to 3rd-level slots. (houserule/clarification, source https://twitter.com/JeremyECrawford/status/596199476202606592)
Tome of Clear Thought/Tome of Leadership and Influence. Add this sentence to the end. 'Tomes of (xxx) have a tendency to somehow wander off/get stolen/be misfiled/etc after having been successfully used.'
Rules and Rulings on Magical items
Attunement: The DMG states that you lose attunement to an item when you die. A bit too harsh for me, instead attunement is lost when you have been dead for 24 hours.
Crafting Magical Items
Do not assume that you can create magical items... (With the exception of Potions of Healing, see General Rules.)
If you somehow acquire a Formula to create one, see dmg128, with the following changes:
You need to have a relevant proficiency. For instance, a character proficient with Smith's Tools might be able to forge a magic sword.
You do not need to be a spellcaster if the magic item does not produce/cast/give you the ability to cast any spells, but you do need to have a strong connection to the item you want to create. (e.g. You need to be a dwarf to create a Belt of Dwarvenkind.) Said connection can also be provided by the proper mystical ingredients. (e.g. The beard of an ancient dwarf shorn in darkness with his own axe while he drank to the health of the intended creator of the belt, praising him in the dwarven tongue.) Research is probably needed...
Tiered Item Crafting (source http://runagame.blogspot.se/2014/12/five-optional-rules-for-your-5e-game.html)
This rule changes the item crafting table (not the cost, just the levels) and makes changes to the rules on how long crafting takes:
The above base costs are general baselines, for specific items, look at the Sane Magical Item Prices pdf (based on these posts). Also note that just because an item has a base price, that does not mean that it is available to buy/sell/make at that price or any other... (See below)
If it comes up, the 'base price' of an item that does not appear in the above lists is usually considered to be at least it's Base component cost, for the purpose of commission.
Selling Magical Items
To find a buyer use the rules on dmg129, modified by the above table, and use the Crafting time for high level characters above instead of rolling for how long it takes to find a buyer.
Buying/trading magic items
If you are in a location where the DM agrees there might be someone willing to part with magic items (such as major tradehubs, great temples, top arcane universities, etc) the DM might state that there are items available due to connections you have already made. (Such as being owed a favour by a mages guild, etc.)
In that case the DM either has a list ready, or might use donjon's magic shop generator or maybe simply create a list of what's available for Common, Uncommon, Rare, Very Rare and possibly even Legendary items.
If the DM does not, he might allow you to make a (depending on desired magic item rarity, starting with Uncommon) DC 20/25/30 Intelligence(Investigation) check to find someone selling magic items of the desired rarity at a cost equal to the Base component cost of the desired item rarity on the table above, you cannot roll again in that location until a period equal to the crafting time for a character of the groups highest level has passed (and items with a higher character level needed for creation than that are simply not available.) On a success, roll 1d3 items of the desired rarity. On a critical success or a success by 10 or more, roll 2d3 items. Permanent items are available for trade, though consumables can often be outright bought.
In general magic items are traded, not sold. Such trades are generally not in your favor...
Normally, you need to offer at least two items of the same rarity or higher, each worth at least the same as the item you want (see prices above.) You may also have to pay a commission to the one arranging the trade (and/or any taxes, bribes, etc, neccessary to set up the trade) of 100% of the combined value of the items you are trading for, or more.
Commissioning Magic Item Creation
It's theoretically possible to commission a magical item, most likely as payment for an adventure. If the creator also demands payment, it normally involves a trade (see above) with a commission, plus whatever the creator's living expenses would be for the time it would take to make the item.