if you have plenty of things to buy, you can gift your items of low value (daggers, goblins bows and all weapons you find basically) to the vendor, one at a time ; you'll see the price of items increase (for you) and decrease (for vendor's items), but I think there is a maximum.
I got from 220% of asked price to about 140% wich is far more reasonable when buying expensive stuff.
Selling price got up from 40% to around 75% too. So you can start selling 100+ gold items with a good profit.
That's if you want to stay without stealing anything of course. Either way stealing their gold is deceptively easy...
Stealing weapons and small items (potions, arrows...) is very easy too, unlike body armors wich are like DC 30. So buying all armor needed before stealing gold and other small stuff is recommended.
