Perfect, i finally have a thread for this idea that ive had for the shops. Costs should go peddler < trader < black market.
A peddler, by definition, is a person who sells small goods. The peddler should sell lower tier stuff (not exceeding 2 color tiers below current color, never higher than purple for when the color after cyan comes out (im thinking ahead here)) for gold (95%) and diamonds (5%). It would be scraps (90%) and every now and again an item (10%).
A trader, by definition, is someone who buys and sells goods, currency, or stocks. The trader should sell things mainly 1 color below your current color, and should offer a mix of scraps (60%) (double of whatever amount the peddler offers) and items (40%), for a mix of gold (65%) and diamonds (35%).
Finally the black market. A black market is a "place" where you can trade illegal or scarce commodities. Black market should offer mostly full items, and it should always be items that you need to promote a hero, up to 2 colors below your current color, again no higher than purple (thinking ahead here again) (80%) and scraps (20%) (3 times the amount peddler offers) for mainly diamonds (75%) and gold (25%) with scraps always going for gold
Also diamond prices need to be lowered