I've owned and setup Tivo's for others for over 15 years and everyone I bought was used! That is inspite of the fact I have little good to say about the company and less with the company that bought them out a few years ago with all of their patents (which is why there are no other true "DVR's" made anymore.)
One unit, no adapters, interfaces, additional HDD etc needed or extra hassles to deal with! A 'true DVR'
There are Roamio's available on e-Bay as long as you look for the '846' series with Lifetime service w/ the plastic enclosure and the separate power supply. There are two other models that are CATV only (840 & 848) that have a metal enclosure like all the former models had and internal PS. Stay away from the Bolt, they run extremely hot and suffer from over heating. They also use the smaller 2.5" HDD which isn't really suited for continuous operation like a full size HDD's (3.5"). The current 'Edge' model, the D6F, I don't know much about, but I believe those use the smaller HDD also.
If you are looking for something less expensive, the previous 'Premier', 746 & 748 series I've seen for under $150 w/ Lifetime Service.
I have owned TiVos since the early 2000's. I even had a Replay TV once very similar to Tivo recording composite video and RCA audio.
I do not recommend the Premiere, the tuner is just OK and being no longer supported at all, I cannot do transfers to other TiVo devices or a computer.
All of my post Series 2 TiVos are Lifetime except one Premiere that is month to month, why I keep paying for it, I must reconsider.
The HD TiVo was excellent tuner wise but they are even older than the Premiere and beginning to have power supply and motherboard troubles.
I have a Roamio Lifetime that is the best TiVo I have, 4 tuners strong signal reception - A #1 in my book.
My Bolts have both died and TiVo would do nothing to help, Weaknees cannot repair them and their tuners were pretty weak.
My Edge has a slightly better tuner than the Bolt but not as good as the Roamio and I do not like the interface.
I have a couple of low power stations 20+ miles away that only the Roamio, the HD TiVo and sometimes the Edge will pull in.
The HD TiVo has a 1 TB HDD inside. At one time I had 5 Lifetime HD TiVos but they have one by one bit the dust after 15+ years, not hard drive failures but motherboard and power supplies that I am not capable of repairing.