Yes, you should be concerned about the valves. But there's pretty much nothing you can do about it (in general) except ask for good pictures and ask specific questions. Alas, often answers are not reliable -- even if honestly provided -- because the seller really has no experience and doesn't know what to say.

I don't think that I would buy what I regard as an "expensive" instrument on Ebay (or from various places) unless I could examine it or had huge confidence in who was selling it (like Dillon) and could talk to them. Otherwise, it's "So relative to how much I'm spending, is it worth taking the risk and figuring that I might have to sink $x into it for repair, or it's a parts horn?" Only you can answer the question about degree of risk-aversion.

Experiences with buying used instruments are all over the place. I give you two examples:

1. Several years ago I bought the 1924 Eb Buescher tuba listed in my signature below on Ebay. Cost was $250, with "local" pickup. "Local" in this case was about a two hour trip there and a two hour trip back. The seller seemed honest and had a good story about why he'd acquired the instrument many years ago and was selling it now. He swore it was pitched at 440 and that any intonation problems could be "lipped". I made the drive, saw an old tuba that looked like a few people had taken hammers to it over the years and the silver plating was black from not being polished for a LONG time. But it looked okay, and I thought I could deal with it. Got it home and polished it up. Looked a lot nicer. Got a magnet and some balls and removed the dents. Looked even nicer. Didn't play in tune. After a month or more of effort, finally figured out it was in fact "low pitch", but "low" meant 435. So I cut it. It still has just a few intonation problems, but I LOVE this horn. It's 91 years old. The pistons don't need replating.
2. Bought a "bass trombone" for $100 (with a hard case) off the site. I was pretty sure it wasn't rally a bass, but hope springs eternal. It wasn't. Medium bore tenor with an F attachment. Broken brace on the slide. Slide seems otherwise okay. The F valve and trigger were a mess. Missing parts and broken solder joints. The case is worth more than the horn , at least at the moment. Some day I may get around to rebuilding the F attachment or removing it and making the horn into just a straight tenor -- then maybe give it to a local school.

The Buescher tuba turned out (for me) to be a steal. I use it for at least some Tuba Christmas events, and I use it for "patriotic" events in my community band. It will appear in a week at the Memorial Day concert and again at our two July 4 concerts. It is a real kick. But I put a lot of work into it myself.

So ... while there are some very general and vague guidelines here, your questions are really pretty much impossible to answer because they're not so much questions about the seller or the instrument. They're questions about YOU.

The problem with buying fairly large instruments on the web is the shipping. Even if you have a warranty that enables you to return it, you'll probably be stuck for shipping. So that's at least the minimal risk.