DirecTv went to a lease model (similar to cable) in March of 06. For the most part any boxes obtained after 3/06 are leases. In some cases the "owner" may not be aware that the box is really leased and needs to be returned to DirecTv and not sold.
If you do find a box that you want to get you will want to get the RID (Receiver IDentification) # off the box. It will be a 12 digit number. Then a simple phone call to DirecTv will get you two things: First if the box is leased or owned. If owned, you can move to step two. Second, is the owners account in good standing? ie, not in collections. If it's in collections, just move on to the next box. If the box is owned and the owners account is in good standings, your almost good to go. The last thing you will need is a new access card for it. $20 and they will ship you a new one.
If your looking at an HD box, the current models that will get everything are H20, H21, H23 for regular HD and HR20, HR21, HR22 for HD DVR. Anything else, skip it as it won't get 90% of the HD channels.
Lastly, you'll also want to make sure you have the current HD dish as well.