Every town should have a designated shop thread with an inventory listed in spoiler tags on the title post. A player can walk in and roll against their charisma to see how they haggle.
It could work as follows: at 10 Charisma, every 5 points on the 1D100 is 1% off. -1 Char, plus one point. + 1 Char, minus 1 point. That’s the base idea, here is an example.
Player has 10 cha, rolls 1d100, gets 100. Price of the item is 20% off. Gets a 50, price of the item is 10% off. Gets a 2, item is 1% off. At 9 char, gets a 100, item is 16% off. Gets a 50, item is 8% off. At 5 char, gets 100, item is 10% off. Gets a 90, item is 9% off. At 15 Char, gets 100, item is free. Gets a 99, item is 99% off.
The idea here is to let players barter on their own and spend their caps without having to tie up an admin. The player, after making the purchase, would DM an admin, the admin would adjust the shop inventory to reflect appropriately, and put the item in the characters inventory.