It is just bad plain and simple. Also regardless if you are considered a "hero" they have a generic 0 attitude towards you. The prices are ridiculous, bare minimum 200%+ mark up to buy and 200% mark down sell is idiotic.
In the 5e player handbook there is no "mark up" buying of items and the only things that sell at 50% are generic (undamaged) weapons. Gems and other trinkets sell for their full value. Magic items are usually only available in extemely wealthy areas and cost is determined on an item by item basis. An enchanted longsword or magic amulet would only be available to the most wealthy (or high ranking clerics or royals) in the land even the upper classes would find these items too expensive to obtain. This is why"adventurers" (you) are sort after and are among the best equipped and wealthy in the forgotten realms.
My point being Larian basically cut and pasted DOS2 trading system into BG3. Again this has been mentioned many times but goblins, even high ranking ones, rarely have generic weapons in good condition never mind magic arrows and spell scrolls. I understand it is a PC game but generic mob monsters are only risky because there are loads of them but are killed by a gust of wind pretty much. A level 1 magic missle would normally kill 3 gobbos where currently you would be lucky to kill one.
Too much homebrew spoils the broth.
