What are the negative effects of selling your blood to a blood bank?

Someone told me about a blood bank here where you can sell your blood twice a week for $40 each time. They take your blood, remove the plasma from it, then pump it back into you. But I was thinking, there must be some kind of negative long term or short term side effects, right? I’m definitely going to ask the place, but I know sometimes you can’t get straight answers from people who want your business..