PC: "I want to sell to sell Rare Widget."
Me: "The shopkeep will give you 1000 gold (random amount that seems appropriate)."
PC: "I have a feat that lets me get 10% more."
Me: "Ok, he'll give you 1100 gold for it."
This is how I run every RPG I GM. I don't see SW:EotE being any different. Might be nice to toss in a paragraph or two for newbie GMs who need everything spelled out, but for experienced GMs I don't see how it's an issue. Just my 2 cents.