Televues are definitely the top of the line in terms of quality. Just keep in mind that a Plossls generally has a field of view of around 50°, which can seem somewhat limiting by the standards of modern designs. I think the reason they are often found in the classifieds is because people want to upgrade to wider fields.
My stock suggestion is always: get a set of Televue Plossls. I'm thinking of adding but to my signature. One thing to consider about frequency in used ads is the reasoning which can often be a result of one or the other: the product stinks, everyone bought into the hype and then got butter and disenchanted with the reality if the limitations. The 70s used ads were CRAMMED with used Questars everyone wanted to unload particularly after seeing a cheaper C8 whip its butt.
That isn't the case with the plossls though...
The other reason in used ads frequency is the sheer popularity of the product where its abundant new and used. That's the case here in the latter.
It could be true that folks are selling off their plossls to fund the very expensive Ethos and and such mega wide field oculars. This is unfortunate however as my opinion is every eyepiece kit ought to have a foundation set of Televue Plossls (though everyone loves Sterling now too).
Between the contrasty planetary views that accentuate the coffee tinge of jupiters belts for example, wonderful clarity and exquisite doublestar views it provides it holds its own place quite well. You don't need to have an 80 degree field of view and all the extra glass to have first rate views. One eyepiece in particular, the Radian always seems to lag behind plossls in planetary contrast while (based on Internet chatter) the Pentax XW seem to have both high contrast and widefield optimized. But at steeper cost than that of a good plossl.
I've enjoyed views through Naglers but Im just not pushed to the great attraction so many find in using these. But that's just me. The ocular forum is a buzz with mega wide angle topics - tis the rage. Just don't lose sight of the excellence had through one of the most successful four element designs ever.