Is there such thing as a test lab/escrow service for selling/buying audio gear?

TMR is sharp.  Two other options are that you could always try selling here or on USAudiomart.

Any big bank or lawyer can function as an escrow agent with a specified technician to evaluate the piece on objective terms. Once the tech verifies, the funds would be released. Whether a seller would want to jump those hoops is another question even if you as buyer are paying the escrow fee and tech fee. I often had exotic cars subject to a pre-purchase inspection at my expense by a trusted mechanic--this is common in car world, probably not so much in audio. 

If you’re tired of doing it yourself TMR is the way to go in my experience too.  You don’t get top dollar but they are great to work with.  I like selling it myself.  It’s a way for an old sales guy to stay in the game.  I like buying used too.  I love doing business for fun.  It’s a fun part of the hobby for me.  

Another vote for TMR.  I have negotiated both price and their fee with them so there can be some flexibility. They have great customer support too. Sent them an amp to sell and during testing they found a problem.  We determined the unit was still under warranty and TMR sent it to the manufacturer for me to get repaired.  They also paid for shipping twice to get the amp back to sell after repair.  Although their fee is high, it does come with amazing support. I have sold and purchased many items with them. 

Maybe check with https://www.escrow.com

It was the first option that came up when I googled escrow services.