Sprint HTC Hero and ESN activation

[likeminas]

Ok. Here's the story.
I had a Touch Pro and I really wanted to try out a new phone, so I went on Craigslist and bought a "Brand New" HTC Hero phone from a guy.
Before going thru with the transaction I requested from him the ESN and proceded to call Sprint to make sure the phone could be activated. After a short 5 min. talk with the Sprint rep. I was told the ESN was "clean" and could be activated immediately.
Having this assurance, I bought the phone from the guy, and later on tried to activate the phone. To my surprise and after reading a few forum posts, I realized that the HTC Hero is not "compatible" (whatever that's supposed to mean) with Sprint Sero plans (discounted plans).
So to avoid having a $300 dollar paper-weight, I posted the item for sale on eBay, where it was sold quickly.
Now after a few days, the buyer has contacted me saying they won't activate the phone.
How could this be, if when I called Sprint everything was alright and told me the ESN was clean and ready to be activated.

Has anybody experienced something like this with Sprint when trying to activate a Hero?

How can Sprint tell you the ESN is clean and later say it cannot be activated?