Hi,
Reseller hosting is a set of allocated resources that you divide and sell to your clients at prices you define.
Here are the most important things you need to look for:
Stability of the company(How long it has been operating, any major hiccups during this time, is it registered company and so on...)
Control Panel(You will need to define compatibility with your billing system)
Server Configuration(You don't want crappy servers)
Looking Glass(You will need to see how quick packets take to reach your server, to get an estimate of loading speeds for your target clients)
Support Availability(In case an accident happens, you need to have it resolved as quickly as possible or you may end up loosing customers)
Price(Prepare to set budget for some time, as you may operate without customers for a while; it will take time for people to discover your brand)