I want the 1st gen iphone, can i buy that then later on put a plan on it?

Like I would buy the phone on ebay then keep it for 2 years then buy plan.

Yes, if you take the phone to an AT&T or Verizon store they will be able to activate it for you.

You can buy any phone on eBay and you won't have to put a plan on it as long as you don't sign a contract with a carrier. This is often a better alternative to buying phones at subsidized pricing due to the money you save in the long run and the ability to switch carriers month to month as you please.