It sounds to me like the seller had used the drive in an external enclosure. He/She probably used the drive as a repository for backups. Maybe they even brought it to their friends' places and swapped files a couple of times. Maybe the seller was/is a service tech who used the drive in an external enclosure and brought it with them around to clients in order to aid in service operations. Who knows but the seller.
Furthermore, it is quite possible that there was no guilt involved -- the seller, in their mind, might just as well as thought that the drive was good as new; and this could simply be because of their subjective determination that it was as good as new as far as they are concerned, or perhpas they forgot that two years ago they ran the drive in a computer for a week.
More likely though, given that a seller is really usually just primarily concerned with getting top dollar, its highly unlikely that they would have admitted to "yeah, I had this thing in an external case and I used it a little bit over the past two years" ... its much more convinent to simply say "never used it" without feeling too bad about it, cause 156hours is pretty close to the truth in terms of hard drives anyways. Its quite possible that they are unaware that power on values are recorded and that their claim can be subjected to scrutiny. Or, perhaps they may be very aware power on values are recorded, but figure there is close to little chance that a buyer would know about those SMART values, so took the chance and said "never used"
Still I wouldn't rule out manufacturer factory testing as solely attributing to those hours. But I would think the values would be reset before shipping....how do manufacturers go about testing drives anyways. Surely they just sample from production runs. So again, I find this explaination the harder to imagine (i.e. what are the chances you actually ended up with a drive that was sampled by the manufacturer?)
The bottom line is this: it was used, but it was a pitiful amount at that. I would just let it go. You roll the dice when you buy off fleabay and, although I know you might be somewhat disappointed that the drive is not brand new, I don't your die came up snake eyes.