If you have more than a handful of domains, then a reseller account is the way to go. I got a Directi account a few years ago at a discounted rate otherwise I'd have considered enom

I have a few other reseller accounts to get the best deal on .uk's and some other types (.uk.com and so on).
If you're reselling hosting or other services, the look for a reseller with an API so you can hook into your billing system.