Cannot really talk about trusted sellers for the On, as I bought mine new and the only used Tama I have ever purchased claimed to be in working condition and I have yet to get it to work, so no experience there!
That said, as far as experience with the devices, I have to say that I adore my pix despite how uninterested I was in it originally. My husband bought me my original one for my birthday last year, and I fell so in love with it he actually got me the party for my birthday this year as well.
It has been my favorite Tama device since the connection era devices for certain. It is pretty close in terms of care and raising Tamas to the older Tamas, which I suppose was part of the appeal for me. There is no gene mixing, no extra worlds in the device, or anything like that. You just have your Tama, and you raise it and bond with it - there is a meter that tells you how much your Tama loves you - and help it get a job, and once it becomes a certain age it leaves. Unless you have truly bonded with it, in which case you can choose to keep your Tama with you for as long as you can. It is simpler than the On in that regard, but it has so many games and features inside the actual device while retaining a sense of old school charm for me that I actually put it quite high on my list of favorites.
For the Digimon 20ths I do like them, they have the old school charm that the rereleased Tamas have, but with an incredible added bonus of having five original versions in one device as opposed to having to purchase one device for every version. I do believe that Bandai dropped the ball on that one as far as Tamas go with the P1 and P2 devices, because I would have loved the same treatment as the Digimon 20ths got there. They do have this one downside that two of the eggs are locked to connections. There is one egg in which you need to connect your device to a particular other device to unlock, and another egg which you need to connect your device to
five differently named devices to unlock. I mention differently named because if two devices have the same name, then they will be counted as the same device. On the flip side, resetting the device entirely and naming it something different will count as two different connections for the device it connects with. So in theory with two different devices you could get that particular exclusive for one of the devices, but not for both. I should also add that these two eggs are single line eggs, meaning there is never any variation in what they hatch into or what they grow into, so unless you really, really want to raise those particular Digimon, it's not exactly something super exciting you are missing out on. Lastly with these you cannot pause pause them, and they poop... a lot. Like... a lot. Be ready for many a cleaning. Bandai made a small error on how often they need to go potty and while it is hilarious it can get out of hand.
If Digimon are something that interests you, there is also the Digimon X line. They are brick shaped like the 20ths, have only one egg in them unlike the 20ths, which in theory means less Digimon, but their growth charts are spaghetti strings of madness that can actually be quite fun if you enjoy a lot of variation each time you hatch a new egg. I enjoy both devices, so I thought maybe mentioning this one just in case.
And to cut this short because I have gone on a tangent, as far as the On goes, I am... meh on it. It's cute. It has gene mixing, which you get by marrying your Tama off to other Tamas. This of course means that you will not be obtaining those classic Tamas outside of your generation 1 Tama, as every other Tama will be a mix. If having the older Tama designs is something you like, this might not be for you. There are worlds you can visit, although one of them is completely locked to having to connect to a second On, which might be a bit off putting for some. There is also the fact that a lot of the officially managed functionality as far as the On went had to do with an app where you could meet other people's Tamas and ask to marry them there. Or wait for special seasonal Tamas and marry those. That app is now closed, so that part of the On's functionalty is officially gone. Again, might be a downer. It has as of so far been the least Tama feeling Tama I have owned. It was cute. The mixing of genes is nice. It just feels more like a different virtual pet that got the Tamagotchi characters and logo slapped on it as opposed to a Tamagotchi itself. At least for me. It's a big favorite for many, many others, so take my critique of it with a grain of salt.
Hope this overly long post is of some use. And very sorry for rambling.
