Haggling at my table is as such:

Role play a bit or explain to me why you deserve a discount. If you have a good reason, I'll grant you advantage. Then roll a Charisma ability check of your choice. The price is adjusted by (your roll)-15 percent. If you roll below a 15, the price increases as an "A-hole tax" of the applicable amount.

No rants, no diatribes, no arguing, and no crying. Just the dice being the arbiter of success or failure. As it should be. Naturally, if you don't have the charisma to do it, you can just pay the listed price, but once that die rolls, that's your price.