Which characteristics of capitalism provide an opportunity for entrepreneurship and allow both buyers and sellers to feel they made a return on an investment?

Price system: in capitalism, prices are set by market forces supply and demand. Prices help coordinate economic activity signalling which activites are profitable, and which are not. An entrepreneur can use the price system to gauge opportunities and take actions, and buyers and sellers use the price system to rationally decide the best course of action.
Voluntary exchange: in free market capitalism with competitive markets, economic transactions are voluntary. This means that neither buyers nor sellers are obliged to buy or sell anything, the only do so if they decide, by their own will, to do it. Entrepreneurs can sell whatever they want, and their decision is shaped by factors other than coercion.
Capital accumulation: capital accumulation is what allows an entrepreneur to start a firm. Without capital, a capitalist business is impossible. Buyers and sellers also benefit from capital accumulation when it allows the growth of enough firms to the point that the level of competition is either perfect, or near perfect.