Can I buy any vintage flip phone?

Hey all,

I have been toying with the idea of switching for a while. I see a lot of people on here buy new flip phones, but i am interested in buying a vintage qwerty Motorola or something like that. I had a phone I liked in 2010 that I would like to find on ebay. Can I just buy any old phone? I have AT&T, and I am hoping i can just throw my sim card in a flip phone and be on my way. Are there any complications that this might cause that I should be aware of?