Current Marketplace for Vacuum Tubes?

Yes no doubt tubes are unique to sell in that the difference between a great tube worth a lot and an average or poor one worth not much in terms of noise and wear, performance etc. can be hard to discern and bank on in advance, so trust is more important than ever IMHO. Money back satisfaction guarantees are probably of value to achieve best possible sale price.

I currently only trust tubes from ARC for my ARC pre-amp enough to justify paying a higher price than I might elsewhere. Especially when it comes to noise levels for use in prime slot in the phono section. That’s one of six tubes. The other 5 slots are way more forgiving. So no doubt "certifiable" high performance tubes in good condition when needed can carry a premium but most are more commodity items still to many.

"I currently only trust tubes from ARC for my ARC pre-amp enough to justify paying a higher price than I might elsewhere. Especially when it comes to noise levels for use in prime slot in the phono section."

You might consider Andy Bouwman at Vintage Tube Services for low noise phone section tubes. 

Hang on to those tubes!  Tubes will be here forever. Quickly obsoleting the more affordable transistor amplifiers and gear. Only six figure solid state can compete.  Brand new tubes are being made, so you don't have to worry about NOS tube supplies running out(unless you prefer them). Personally, I like VTL(Vacuum Tube Logic) gear. Rock on!

Listed tubes on US Audio Mart, AudioAsylum, and a few on E-Bay.

Thought I’d follow up now that I have marketed and sold a dozen pairs of vintage tubes.

Audiogon ads received little action, rendering the cost of a basic ad plus photo at $9 a poor value. I spent $45 on ads for items that didn’t sell (and received no offers).

I sold two pairs of tubes on E-Bay. The cost of listing, final value fee and Paypal fee at an aggregate of about 11% is another relatively poor value.

The remainder of the tubes were sold via AudioAsylum Trader or USAudiomart. Considering both those marketplaces cost nothing to post ads, and one can accept several forms of payment, including Paypal "Friends and Family", they are the current go-to venues, IMO.