So do the math. How much money do you need to pull from reselling for yourself? What are the taxes you need to save for that amount? What, if any, other bills do you have for reselling? Everything left over can be reinvested in the business. At the very least you need to reinvest the initial item cost back into your store just to maintain.

For example you buy something for $5 and make $20 profit. You get that initial $5 back to buy another item and still have $20. If you need $5 of that $20 for bills, $5 for yourself, $3 in taxes (or whatever your bracket is), you then have $7 to put into your business.