Fey trade house

If you can find a copy of it, I found a great resource as the Thrift Store years ago. It is called "The Heroes of the Feywild". It has some race and class choices, but what I found most beneficial was the sheer amount of lore and locations of the Feywild itself. For something from actual 5E content, the Eladrin in Mordenkainen's Tome of Foes could also offer much inspiration.

In my general assumptions of the fey, they are fickle creatures that are neither benevolent or malevolent. Some quirks or traits of they fey may be mischievous, cunning, elusive, spiteful, vengeful, charming, eccentric, unpredictable, etc. Also, the denizens of the Feywild tend to be divided into two courts. The Seelie (Summer Court) and the Unseelie (Gloaming/Winter Court). Maybe the purveyors of this trade house change to the Unseelie when night falls or winter rolls around? Then during the day time or summer time the Seelie court run it.

Another aspect of the fey to think about is that they tend to value debts and favours far more than material wealth. Perhaps some of the peddlers do not deal in any material currency at all. Instead they bargain through favours that may be asked for immediately or down the line. Not all Fey care about favours, but it is a common aspect of the fair folk. Fey are typically very true to their nature. The best way to remain true is to have a few key characteristics that embody them, a member of the Seelie Court may be described as amicable, mischievous, as stoic as the mighty oak, and never forgets a slight against them.

Do hope you were able to find some sense in this nonsensical rambling and keep on Dungeon Mastering!

Well, in Irish folklore (as well as 4th Edition), the fomorians are considered natural enemies to the fair folk. With that being said, if there is any monster that may interest you or feel would be a good antagonist then weave your own story for them in your world!