If a domain is newly registered and still under the 60 days ICANN rule, it must be pushed to another account within the same registrar. So if you register a name at Enom and sell it within 60 days, you can only push it to the buyers Enom account.
If you buy a domain from someone and it is outside the 60 day rule, they will still push it to your account from the same registrar, but then you can transfer the name to another registrar (if preferred). The only exception to this is GoDaddy which requires a 60 day hold on a name before it can be transferred out after the domain contacts have changed.