Tons of them on Ebay, and asking prices are all over the place.
You should definitely get the serial number on any prospective purchase so you can check with DTV to ensure it's not either a leased unit, or still tied to someones account (That CAN be an issue). And the client WILL have to purchase a new access card from DTV for $15,00 or so before it can be activated.
Last, but not least, those old HD's are very prone to having problems so you may want to recommend a new, and larger, HD.
Here's a couple:
[Link: cgi.ebay.com][Link: cgi.ebay.com]FWIW: Don't believe the claims of "tested and works good". Just because a unit will power up and you can watch some off air stuff (if it has the old card inserted) doesn't mean the unit will work properly with DTV/Tivo service.
It can be a crap shoot....