Just look for the higher value Free Agents. They often are decent enough to play on your team (and will have a super low salary!), and if you sell them on it's good $. It is pretty random when they show up, some are generated but the majority are sacked from higher level teams.

Selling 3* guys at lower levels *is* difficult, you may find you need to sack some of them if no one buys 'em.

I pretty much have 21 or 22 players, even if you are only in two competitions it helps to keep everyone in better condition, and for covering injured guys. You are going to want to evaluate what roles you only need one or two of, and which (like MC) that you need more. This is where having guys that can play two roles is helpful, especially since you only have 7 slots on the bench to cover the field.