Buying a cellular iPad second hand (New sealed)

[Faded]

I've noticed quite a few situations where sealed cellular iPads are popping up on Marketplaces.
Why would people get these on contract and then sell them on? Is it something to worry about should they not pay their contract? Any other concerns these might come with?

 

[Faded]

I did worry about that but a couple of people have posted them alongside receipts.
I was wondering if they could maybe using the sim for other purposes and just selling the iPad to make some money

 

[Faded]

So what issues could come of them not paying their contract? Could the network potentially block the iPad?