They will give me also the html code to add it in my site? I must also create a panel or something so the users will add there the nameservers? Thanks
Enom requires a deposit. GoDaddy has an annual fee. NetEarthOne requires no deposit but you can get better rates if you do. You can also get an account from an Enom reseller (like domainsmarty.com) and not pay a deposit. In all of the above you would have a separate site (a separate domain or a subdomain on your main site) as they provide a page template that you can modify/customize. By using their API tools you can integrate directly into your site but that does get complicated with room for error. Better to use their hosted template solution
ASTRAPI, Check our domain reseller program .. You'll be offering domains for your customers in no time
I recommend ResellerClub..Enom is the next best option..Godaddy Reseller programs are not that great..they are excellent when it comes to end customers but not reseller programs.. I personally use ResellerClub for all domains and for those domain extensions that are not with RC, I use Enom..