Phone and aircard compatibility

I have Sprint, cell phones and aircards for the company. One of the aircards needed to be replaced. ($231!) I can get one off of Ebay for $65.

My questions are these.

If I go to Ebay or an equivalent, are the aircards going to not only be compatible (easy enough to tell from model numbers) but will Sprint activate them to my account? Or are they nasty about it?

This goes for phones as well. If I pick up some Sprint compatible phone through other means, will they activate it? How do I make sure that I am purchasing something that they’ll accept?

Is this a complicated process?

The phone I have presently does not have a SIM card.