I'm not sure what they mean by resetting it to factory conditions. There's a way to reset software (copying over the original software image to the memory stick using a Sony reader/writer), but I don't know of a way to reset the hardware of a 210 (there could be some way I don't know of though). With 210s it's best to see a video of the robot in action since not all sellers even notice TAS or jitters, but if the person seems trustworthy, it's alright to just take their word for it.
To get a good idea on prices, I just sold a 210 yesterday with a box, charger, 90+ minute battery, pink ball and Aibo Life software. This ERS-210 was pinned for TAS and PAS, but not DHS, and sold for $500 after being listed for about two months. I'm not sure what a fair price would be on the one you're interested in since I don't know what other accessories it comes with, but I'd guess under $500 because it does not come with official Aiboware and has not been pinned for anything (it could be more depending on what extra accessories it has though).
The ERS-210 doesn't have as fancy tricks as the ERS-7 does, but the autonomy is better. A 210 will not be able to balance a pink ball on its back, but I would say it is more "random" than an ERS-7 (a 210 will be much more active when left alone in a relatively quiet room than a 7). ERS-210s also have more advanced "learning" than the ERS-7s do since the 2X0 series remembers what it's scolded for and what it's praised for and bases its actions off of that, whereas the 7s don't. Dogslife does have more silly skits and random dances than official Aiboware and I think you would find a 210 running this program as quirky as an ERS-7 is (there's also the "Show off" trick in Dogslife where the Aibo will play a skit at random). I don't currently have any 210s running Dogslife, but I do have
this really old video of one of my ERS-210s that used to run on it (I apologize for the shakiness, the video was taken almost four years ago before I had a camera stand or knew how to properly take a video with an iPhone). Another nice feature of Dogslife is that you can create your own skits or dances with Skitter and then add them into the program (triggered by a voice command, touch sensor, etc.) to customize your Aibo's personality (
here's one of the 210 dances I've made, only the dance is by me, the vocals are by rockleetist).