There's nothing to buy after I've bought (and fixed and sold) the first piece of pixel art?
Which version were you playing (Windows/Mac/Linux/Web)? The game goes to a new "round" every minute (indicated by the countdown timer that appears on most screens). At the start of each round, 0 to 3 new pieces of broken pixel art are generated, but if the store is empty, then at least 1 should be generated.