If you are buying a domain that has just expired and are trying to buy one with PR there is alot to actually ensure you are buying what you think you are buying. A few (of the many) places that you can use are:

http://estibot.com/index.php
http://smartpagerank.com/

These are free, but I am sure there are more reliable places you can pay for. The Google toolbar can be used, but beware of fake page rank.

Hope this helps.