I'm selling my old phone and I'm the original buyer. The ESN should be clean right?

I would have to say yes

ESN would only not be clean if the phone was reported stolen in the past. Trust me, you’d know if that happened. Because it would be blocked from the network and you wouldn’t be able to use it.

You can call your recent phone company and they will tell you if it’s clean or not. I have bought a couple of phones on eBay and called the carrier (virgin Mobile in my case) and entered the ESN number that was listed in the listing. They were able to tell me if I were to buy the phone if I would have any problems connecting it.