The cert is only valid for 6 months and RA runs for up to a year (depending on your Ambassador expiry) so you can only refer yourself if you run multiple accounts.

Note also that PC occasionally randomly asks people where their cert came from and will not process it until they have had a good response which does not include the words "I got it from a referred RA". This has happened to me. It is a small risk but, if you're buying off ebay, note that it is a risk nonetheless.