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

In my experience TMR pricing is pretty accurate.  It's possible what you're hoping to get is just beyond what the market will bare.

I agree the 35% seems a bit steep but in all honesty their service is excellent.  I've used them several times, they covered all the shipping, testing, photography, advertising, working with the buyers, collecting the money and promptly sending me the proceeds.  Twice they've even provided me with boxes for equipment I didn't have the original container for.

It is a bit of a premium service, but they deliver it well.  It is very much the service you described in your question.

At TMR you get 65% of what they sell it for and you have to do nothing but box it and ship it to them, and they provide the shipping labels. I did that with a tube amp and pre amp, it went well for me. They test everything before they sell it and they provide the new buyer with a trial period. If you don’t want the hassle then you are not going to get top dollar too. 

+1 for TMR. Recently sold a phono stage through TMR. It sold within 3 weeks for listed price and had my money within a couple of days. They made sure it was in good shape and ready to sell. They made shipping easy. The best part-I never had to deal with a buyer and don't have to worry about an unscrupulous buyer causing trouble after the sale. The commission is a bit stiff, but worth it to me-I'm a consumer not a dealer. I'll use TMR again. 

Try only excepting a gold bar as payment! 

Matt