To choose a Domain Reseller,what is the most important to consider?

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Joined: Nov 2013

If I'd like to build up a large website,which contains huge Web space,more than 50 email accounts.Of course,I need to register different Domains. Should I choose to buy one by one?Or I apply to be a Reseller,buy all in one time.Which one is better?
If I'd like to be Reseller,which company is more competitive?