I am using Trials of Tav along with Trade Withers and all of the ancient equipment mods. I wanted items from the mods to show up in Withers' stock as well as be removed from his stock on a long rest. Non-magical weapons, armor, and jewelry will not be added to Withers' stock, because nobody wants that useless shit. I thought it would also be more fun if legendary rarity items could only be obtained from drops or the unlocks menu for Trials of Tav, so I excluded those from the equipment tables used to populate Withers' inventory.

HOW IT WORKS

1. There is an equipment table for each category of items: armor, weapons, jewelry, potions, scrolls, arrows, and camp clothes. These tables contain ID's for base game and ancient mods equipment. Giant lists of UUID's
2. Upon starting a new game, Withers' inventory will be populated
3. 10 random (but different from each other) items are chosen from each category
4. Legendary items will NOT be chosen.
5. Non-magical equipment items will NOT be chosen
5. Each chosen item is added to Withers' inventory in varying quantities.
6. Equipment items (armor, weapons, jewelry, camp clothes) are added in stacks of 1, while consumables (potions, scrolls, arrows) are added in stacks of 5
7. After a long rest, Withers' inventory will be wiped out completely and repopulated in the same manner as described above. 
8. Leveling up will NOT reset Withers' stock
9. Loading a saved game will NOT reset Withers' stock
10. I've added a tag to Withers to prevent him from being pickpocketed entirely, as I noticed in yakuzadeso's original mod, it seemed Withers could still be pickpocketed in multiplayer by one player while engaged in trade with another. 
11. This mod deletes EVERYTHING from Withers' inventory after a long rest, so items from any other mods can be safely sold to him.

REQUIREMENTS

Trade With Withers, Ancient Armoury, Ancient Weaponry, Ancient Jewelry, and Master's Cloaks are REQUIRED. norbyte's Script Extender is also REQUIRED. If you do not have the equipment mods, this mod will try adding non-existent UUID's to Withers' inventory. I may make a version that only needs Trade With Withers (and script extender) if enough people want it just for the modded item deletion on long rest without needing all of these equipment mods.

Trials of Tav is not required for this mod, but that's the mod I made it to be used with. I find the mode more fun with a trader and it gives a nice way to get rid of all the loot you pick up. From my understanding, If you are not using Randomized Equipment Loot, you will effectively have no way to obtain the legendary items from the ancient mods unless you download the cheat versions, as the items will not drop randomly

THINGS TO NOTE

With all of this in mind, if you install this mod on an existing playthrough, you will not see Withers' new stock until you long rest at least once. If you would like to reintroduce legendary items to Withers' stock, you can simply unpack the mod and uncomment the legendary UUID's from the item table lua files I've added. I designated each commented out legendary item as such. Additionally, if you would like other modded items to appear in Withers' stock, you should be able to simply add whatever UUID's you want to the appropriate equipment table lua file. 

Since the clear inventory mechanism I used does not need to be given specific UUIDs, Withers' inventory should, after a long rest, be cleared of EVERYTHING sold to him, regardless of whether or not the items exist in the lua tables. This means you can sell any modded items to him and not worry about them clogging up his inventory forever. 

A couple extra notes: 
Cloaks are a part of the armor table, and the Master's Cloaks mod adds 100 of them. This will mean there are more cloaks than any other equipment slot in the table, and Withers may disproportionally stock cloaks over other armor slots. 

Shields are a part of the weapons table, so if you do not see 10 different "weapons" in Withers' stock, it's probably because half of them are shields. 

P.S for people more familiar with making mods: If you look at my code and find that I vastly overcomplicated things, please let me know. I was very surprised there was no simple "Clear Inventory" function. I also considered that it may be easier to somehow make withers just use treasure tables like base game traders, but from what I could tell, base game trader logic does not delete items sold to them after long rest, and also I haven't really learned how to write mods outside the script extender yet, so it's probable that the no pickpocketing tag I gave to withers can just be a static change in the game files, instead of setting the tag every time a save is loaded like I've done. The mod also uses a listener that listens for "EntityEvent", which I am guessing happens a lot. I would rather the listener I set up be unregistered when it isn't needed, and I've tried to do that in my code, but I'm not sure if it is actually unregistering it or not. 

Credits to:
Paramonov95 for his equipment mods: Ancient Armoury, Ancient Weaponry, Ancient Jewelry, and Master's Cloaks

yakuzadeso for their mod Trade With Withers

Gigadroid for their mod Trade with Withers - Magical Items only and more random selections, I used these item lists that already had a lot of stuff commented out for me