On a reseller business. What percent markup is profitable to use as a formula?
After all you cost is realized to bring the product to sale.

That is up to you. You have to be sure to include ALL costs to you - when figuring what to resell something for. Some people like to double their money, others are happy with just 10-20%... but the key is making sure that you account for all of your expenses when considering what to re-sell the item for.

The majority of companies I have worked for do this. If they buy the item for 100 dollars, they sell it for 200. They are not double their money since they still pay for the store, salaries, electric and all business costs out of the 100 dollars over the cost of goods. So they have to control that cost. That is the issue, controlling the other costs, If they spent too much, they lose money, if they keep costs down, they often ended up with 15 to 20 percent total profit.

Many business work with about a 10 percent total profit.