We have accounts with enom, godaddy (wildwest domains) and bulkregister. OUt of the three bulkregister is the most exspensive and least versatile. If you are looking for an API (application programming interface) which would allow you to sell domsins from your site and charge them with your merchant account then enom is what you want. They have the most tools for hosting companies looking to dive into the domain business. Godadddy also has an API but it just came out this year and is more money up front and not well tested yet.

Another option is singing up for a H-Sphere reseller program which you can sell domains through your hosting reseller account. This also works well. The down side is that you have to use their API to sell domains but if you can find a H-Sphere partner that has a low rate on domains it work out well for you. If you start your own cluster with the H-Sphere system you can use your own enom account, either way it works great and easy to maintain and set prices.

You can find out more info on both of these services on our site. I hope this helps in your quest.