What is the proper, polite thing to do in this situation?

I sold some books online, and the people who want to buy them are coming to my house to pick them up. I'm only 15 and I've never done anything like this before. My parents don't want to help me, so I'm doing this all myself. So what do I do when they come? Do I invite them in? Or do I just say hi and then get the books for them while they're at the door? What do I say afterwards, "I hope you enjoy the books" or something like that? Please explain to me what I have to do, I'm absolutely clueless! Thank you in advance.