I have bought all of my NOS/NOB/USED vintage valves from Ebay (easily 100+ valves)
I have only had one dud (an RCA 6V6GT) which only cost me $9 plus shipping of $8 , so it wasn't really any form of kick in the guts. And since I now have my own valve tester, I feel I have done quite well for myself over the past 3 or 4 years.
Brad's advice is spot, but I would also like to add the "get a bargain" methodology. I'll use the following pic of my 6V6 stash as an instruction tool:
There various types of sellers of valves on ebay:
(a) The person who goes to estate sales and buys some old (dead electrical/TV repair man) guy's stash of valves. Then sticks them on ebay without any info or testing.. usually just a single pic. Or someone who finds old (NOS/used) in their parents/grandfathers whatever attic. this is where bargains can be found. you need to keep a watch of ebay. I keep clear of any auctions from these sort of people where they say the valve is used AND there is no box that looks like it is correct for the valve.
However if there is a photo or two which shows the box in good (even if it's dented) condition and shows the valve and the printing on the valve is perfect (not rubbed off) then this is worth looking at . Remember it is always a risk, but as I said I have only had one dud in greater than 100. If the pins look shiney without gouge marks, if the glass looks really tranparant, there isn't "burn" marks on the inside of the glass... then these are all good signs. This is how I bought that box of 4 GE 6V6GTs in the sleave.... ended up being something like $15 each. I also bought many of the single GE 6V6s and RCA 6V6s that way.... just someone with no knowledge who has found a genuine NOS 60s valve somewhere in original box and then stuck it up on ebay for a ridiculously low price.
(b) Similar to (a) but they have some knowledge of the demand for certain valves and usually put the price way too high. Often these people have "fallen over" a stash of valves and to them it's a gold mine just waiting for them. I bypass these.
(c) Then there are the sellers who have a stash of valve and have test equipment (as described by Brad) .. these people put valves up on ebay all the time... hundreds of valves... most times these valves are used, without boxes. They do hawever stick some sort of testing data... which in a lot of cases is meaning less (some valve testers give "good" info, some just give info that allowed TV repair men to know when to replace a valve. These people I have bought from, but only after I have check out all their feedback and watched them for weeks, months on ebay. There are plenty of people who sell like this. and again bargain can be had sometimes because there are a lot of used, but still good valves out there and often more than those who want to buy. Make these people an offer on valves that don't get sold.
A couple of days back I made an offer on 4 6V6s to a guy with 4 RCA for sale... this is his relisted auction. he wanted $200 I made an offer of $75 (ok that would be $17 each tube... not bad) He rejected the offer. Why did I only offer $75 ? well I don't need them (but my guitarist getting a DR so he might) but also the seller says
"..all 4000+gm.3700gm recommended value..tested above nos level on the hickok 600a mutual conductance tester." Well this plus nowhere does he say NIB or NOS tells me they really ahve been used. I have NOS RCA 6V6s that read over 6000 on the Hickok 600A .. so reading only 4000 is way down from NOS ... it not exact numbers, but it gives an indication. So I wasn't annoyed when he rejected my offer... he now has them up for sale starting at a very low first bid.
here's two auctions that I did win recently
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... MEWNX%3AIT" onclick="window.open(this.href);return false;
http://cgi.ebay.com/ws/eBayISAPI.dll?Vi ... MEWNX%3AIT" onclick="window.open(this.href);return false;
both from the same seller .... so three 6V6s for $25, whichg is $8 each. Postage is cheap, the seller has sold a shit load of valves over the years and has a good ebay rating. Plus the valves looked ok... printing on the valves looks good
So I took a risk and I shall know in a week or so time whether is was a good risk.. I think it will be.
It doesn't hurt to "ask the seller a question and make an offer"
(d) Radio geeks .. there are radio geeks out there who are not into guitar amps and they have a shit load of valves (I've been to swap meets here in Oz and they make our geekness shrink into nothing) These guys are often middle aged or old and really just want to thin out their collection of valves and usually do not have extremely high prices... The two white boxed 6V6s came from a radio ham guy who had tested the valves correctly, written all the values I, voltage and Gm on the end of the box ... again check theior feedback out like you would with any ebay seller.
(e) professional guitar parts shops like angela, KCA ... I tend to steer past these guys on ebay because with the "blood in the water" mentallity of some ebay buyers I would rather buy a set price from the guys from their actual online store
(f) Foreign - none USA based sellers. Lots of ebay sellers who are in say
turkey:
http://stores.ebay.com/The-Tube-Sound" onclick="window.open(this.href);return false;
or russia or taiwan or germany ... et al ....
as Brad said elsewhere, there is a shit load of hardly used military valves that have been on sold to people ... boxes of the stuff and these strang (to me any way) countries have them. Again keep a watch and bargains can be found. I pickewd up 2 Mullard 1961 NOS NIB GZ34 rectifer valves 12 months ago from a guy in Russia for about $30 each
Lots of guys in Germany with great test equipment selling excellent used valves on ebay.
(g) lastly there is the HiFi expert who wants to hear James Last in perfect stereo sound on a scratchy old LP from the 60s. these people buy and sell valves like you and I buy beers... buy a match set of power tubes one month, play them for an hour and then think they can do better with a different brand and then sell the first set on ebay. Pick up some EL34s that way recently
Basically just keep your eys open... start to "read' the signs of what you think are honets sellers and who are dodgy people and you should be ok.
but remember DON'T BUY THE FIRST THING YOU SEE ... do your homework
relaxing alternative to doing actual work ...