Buy-and-sell aka flipping houses is not really investing in the true sense of the word. Flipping houses (buying and selling within 1 year or so, hopefully 3-6 months) is a semi-active source of income and is much more like a business than buy-and-hold is an investment. The basic goal with buy-and-sell is to buy cheaper than you sell for and to make a profit. Between the time you buy and sell, there are generally costs (expenses). Those can be holding costs (utilities, insurance, taxes, other payments) and construction costs (labor, materials). When you go to sell, you have selling costs too (excise tax, title insurance, escrow/title fees, real estate commissions, etc.).
To be successful at buying and selling you need to buy low, sell high and have less costs (expenses) than the difference between what you bought it for and what you sold it for. The remainder would be your profit!
But don’t forget the taxman, too. He’ll take his cut out of those profits too!