the shops are 10:1 at least. is there a good merchant with good sell price in the game?

Some trophies give you extra cash. Most shops purchase things that are relevant to them for higher prices than a general merchant would. But the best, and fastest, way to make money is to do quests. Selling stuff and junk to make money is trickle income. Don't forget to collect every Florin and Orin you find on your travels, and cash them into the bank in Novigrad.

Edit: Oh, and not a smart subject line. Sounds like spam.

try to sell things to the appropriate people; sell swords and weapons to blacksmiths, armour to armourers, food and drinks to barkeeps etc. They will pay more for the products they deal with and give you less for what they don't ie: selling armour to smiths will give you less money than if you sold them weapons. another tip is to look for hollow mossy logs laying on the ground- they contain only a little bit of money but every little bit helps. but questing is the best way to get coins.