Providing the ESN is important (for the purpose you mentioned), especially if your seller feedback is scant or if it's a newer, more expensive phone.
One solution might be to say "ESN provided upon request" and let people e-mail you for it. Some people do disclose it in their listing, though there is the off chance that some dumbass will add it to their account without buying it. Can be a hassle to get it cleared off.
But if I were shopping for a phone on ebay and the seller had 50 or more pos. feedback and stated in the auction listing that the ESN was clear I'd have no problem buying it without checking the ESN.
l2c