Guessing Activation lock email and password?

The number of characters in the obfuscated email address is always the same, regardless of how long the actual email address is. So that approach is doomed. For the mathematically challenged, even assuming that the email address is 5 characters long and it can be any combination of letters and numbers (at least it is case-insensitive) each character could be any one of about 40 characters (including allowed punctuation). So the number of possible email addresses, even for just a 5 character email address where you know the first one is 40*40*40*40, or 2,560,000. If you could try one per second it would take 711 hours of guessing or 30 days if you were at it 24 hours a day without sleeping or eating. If the email were only one character long it would be 102 million possibilities.

To date, no one has been successful at bypassing the activation lock. You're not going to be the first by simply guessing.

The likelihood that you would guess both the correct email and the correct passcode is incredibly small. As the seller was upfront about the fact it was iCloud locked, you're not likely to get your money back from eBay.