Will Verizon let me use an iphone from someone else?

So, I have a blackberry and I have had it for about a year now. I've been wanting an iphone and I plan on maybe buying one from someone on Ebay or something since it's a bit cheaper. Would Verizon let me use an iphone that I bought from someone and not from their store? And would they let me change my plan with it since mine is for a blackberry?

Woah don't buy from ebay, lots of people who buy cheap iPhones end up getting fake ones.

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