Just some sum-up which is mentioned above...
Requirements for reseller:
1) adding new HW (especially if you will resell VPS)
2) support (24/7 vs 24/5?) -
the most important one (you really don't want a support which works in work-week only! Your customers work 24/7)
3) locations of their servers (US vs EU vs Asia - it depends on your needs)
Just send them email that you are interested in reselling their services and you will see how things work
There is a good way to test your chosen dedicated server for a while. A trustworthy provider should offer a trial (even though it does not cover all setting fees). You could directly ask a provider if they can offer you server for week / few weeks for a better price just for the test. If your business model makes sense to them I don't see any reason why they should refuse your demand.