The conventional approach in the computer business is to make the machines first, then sell them. But a Lexington start-up, Adapteva , has rung up nearly $900,000 in sales of a computer it hasn’t yet made.

“We have a market. We have a customer base and everything else,” said founder and chief executive Andreas Olofsson. “The stressful part is we’ve already taken their money and now we have to go build them.”