If you need advice on tubes, Ebay may not be a good route. At the same time buying from reliable sellers that test carefully, can talk about the tube sound, have retail outlets, websites, etc, this costs money. For example, I have a lot of tubes as good as those sold by Upscale that were way, way cheaper. But, if you need the support, they, and tube sellers who are careful about their testing and listening impressions, can be valid resources for sure.
Similarly, Decware prices tend to be higher than ebay, but talking with Steve, higher cost comes in part from all the tubes in batches, new and NOS, thrown out, not fitting his standards.
If you want to "branch out" though, perhaps having experience with tube types, companies and vintages....and/or you know what you are looking for, having liked reports from reliable opinions about specific tubes, Ebay to me is a great place to get tubes. I have bought almost all my NOS tubes from Ebay Sellers, sellers from all over the world. But I am pretty considered about it.
First, Ebay needs good sellers for Ebay to be trusted, and gets rid of bad ones. To this end, they will also help resolve issues with sellers if they come up, including refunds and mailing if needed.
But it really helps to know what you are looking for, and at. There are tubes up for sale where the seller knows little or nothing about them.
If you are unfamiliar with looking at tubes here are some pointers:
1) do an Ebay search for the tube. Then check out the ones that look straight and clean, have good descriptions, and look like decent values. I open each interesting tube in its own browser tab making comparison easier.
2) check out the seller ratings...If there are problems there move on. But if there are a fair number of ratings that are mostly, or all positive, probably a good seller.
3) How are the test scores compared to NOS? If not stated, or you can't discern from the scores, ask the seller?
4) If you need matched pairs or quads, you can add those words to the search if you want, or just scroll through the broader tube type search...sometimes you will find pairs or quads that do not show up in searches using those words.
Is there test info? Are they relatively closely matched? If you can't tell, or they say matched with no scores, ask the seller...how well they match? How do the scores compare to new?
Also....sometimes the seller does not have a tester and is a big tube enthusiast themselves, wanting to sell part of their collection...Often they know that the tubes tested good before they bought them, and the approximate hours they used the tubes...Low-use tubes from this type of seller, in my experience,
can be really good buys. Do they have good seller ratings? Does what they say feel real, and does the price reflect lack of testing assurance? Near-NOS tubes, like good testing "pulls," can be nearly as long-lived as NOS; they have worked in test equipment (or someone else's amp in this case); and are usually notably cheaper than true NOS. I like this.
Also, at times the seller is not a tube person...selling dad's tubes, or from estate sales.... These can be great buys, but riskier...However, if they look brand new, they might be worth checking out further if the price is right, especially for single tubes where a match does not matter. And check with the seller, will they refund tube cost and shipping if the tube is not good? I recently got a new looking Mullard made GZ32 this way for about 1/3 of the less expensive side of market price.
Also, maybe just lucky, but I have found that five packs of brand new input tubes from the big European NOS tube makers (Phillips, Mullard, Brimar, Amperex, Siemens) tend to match scores pretty well. Maybe I have just been lucky, but this has been the case for me several times, and these packs of tubes are often quite inexpensive.
5) If looking at pairs or quads, I look carefully at the construction, making sure they match in construction.... This can be fun with some practice.
Though not always critical, if the dates, construction, and scores are matching, the quality and sound is likely matching.
Branching out from pretty complete matching a little can be rewarding, but is more tricky. Or you can get lucky. A really good buy, I ordered a "matched pair" of Mullard E88CC gold pins from a big audio parts seller a few years ago. Great score match, and many things were the same, but others different. The getters were the same "parasol" type, but one on a taller riser, and one short (also making the getter flash of one higher in the glass). And the micas were different, one tube having squarish micas, and the other round with regular points going out to the glass...the tubes were different productions yet called a "matched pair."
I got ready to complain and send them back, but decided to give them a listen first. At first they were a little rigid and thick sounding being truly new, but they showed all the signs of being beautiful sounding tubes. And switching channels, they sounded very close to balanced both ways. This is very unusual for me to buy or accept this poor construction match, but I have been using these tubes most of the time in the power position in the CSP3 for a few years. And I have many relatively perfectly matching tubes I could use.
Still, I prefer the security of known matches in score and construction when buying tubes.
I look at the glass and base...shapes and colors....mold ridges on the glass tops on tubes with the little nipple....silver getter flash in the glass...Is the flash in the same area of the tube and about the same height, diameter, density etc...
...the plates shape, color, and hole pattern (or lack of).
The mica shape and placement with the plates...between mica and plates setup...same?....the vertical proximity of the micas in the glass...the mica thickness and opacity...the edge shapes going out to the glass....holes or cutouts in the micas ...
The wire colors and arrangements and other parts between pins and the bottom mica or plates (some tubes have no bottom mica, instead having a glass "blob" the wires rise out of, the wire going right into the plates)....
The parts used on top of the top micas, colors, arrangements....
The getter riser and getter shape and size/diameter...round/hallow getter, D shape, parasol...look at the arrangement of the getter...do they look alike?
The writing on the glass ....
Looking carefully at whatever is there to see with comparison in mind....
Or if you feel like you can't quite tell, ask the seller. If they know tubes, they will very likely tell you if construction matches.
5) getting more specific, are the date codes or stated dates of construction close or matching?
Not absolute, but these sorts of investigations have proven good for me as a tube... uh....junky