I would recommend you start to offer shared hosting (web hosting) without reseller option first. Once you think you can keep server stable with quite large number of website. Start to offer reseller. Do not allow reseller client to oversell as this may caused server problem easily.
Dailyrazor.com offer nice reseller hosting deals. They are very quick to respond to help desk tickets!! I had a problem after updating a script on the site and they were able to restore the site from a backup very quickly!!! EXCELLENT SERVICE!
Usually any good webhost would be happy to help their resellers in every matter. Can you please share how long are you with them and your website address? Thanks!