As in any business, the sale price of an item is directly related to the wholesale price. Wholesale price plus profit margin equals retail price. If you can make 30% to 50% on an item you are doing very well. For example, if you buy 5 watches at $100 each wholesale, you should sell them for no more than $150.
If you are selling a used item in good condition you should discount it at least 20 to 25% from what you paid.