The shopkeeper stuff is really badly implemented, but here's what I know
Higher charisma will lower prices of items you buy and increase the amount you get for selling
The character you talk to the vendor with is the charisma that affects your prices - you can switch do different characters inventory to sell from and it still uses the character you talked with. For me that's Wyll
You can donate gold to the vendor to improve their attitude towards you. This is done in the barter screen by just dropping some gold in the trade window and completing the trade to give it to them for nothing in return. The amount approval changes by depends on the level of the character you did it with. At level 1, it costs 600 gold to max out and get the best discount. At higher levels, it goes up steeply and is never worth it. So the best time to do it is if you just respecced someone. You could use a level 1 party member that sticks around camp that you don't use, but the attitude only applies to the character that did the donation, so you'd have to talk to the vendor with that character. The max attitude bonus savings are pretty decent if you are buying items that cost thousands of gold. 600g could be worth it sometimes, but often isn't. It's never worth it at the higher level costs.
It seems to reset this on attitude bonus on long rest. This might be a bug, it sounds like it should persist once you've improved relations with a certain vendor
As another note, vendors reset their gold on level up. If you've brought a vendor down to zero gold, it will give them back a decent amount of gold. But also if you've sold a bunch of stuff to somebody and they have a ton of gold, that will disappear after you do a level up, so be careful if you were planning to sell a ton of stuff to that vendor and get it back.
Also, as you get higher level, vendors will get new stuff, so it's worthwile to visit vendors and see if they have any cool new items that fit your builds that you'd like.
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The batman villians all seem to be one big joke that batman refuses to laugh at - SantaRPG