I've got the thieves guild skill line and got haggling to stage 4. This would make items worth 10% more, but does not apply to laundering.
However, when I checked today, it actually does make my items cost 10% more to launder.
Did you actually launder an item to verify you’re losing 10% more gold than you should? I mean to rule out a UI error like the one where Rolis Hlaalu says he’s giving 0 writ vouchers but does actually give the right amount.
I am attempting to test this but can’t replicate the issue even in the UI. I have 4/4 Haggling but the fence still only wants 1g to launder Nickel, 100g for a green treasure, and 9g for a green recipe. He will pay me 110 for the green treasures if I fence them instead of launder.
I’ve found that Master Merchant will cause this if you set it to replace game prices with market prices in inventory. But it doesn’t actually change the price charged, so it’s safe to launder.