What is considered a "standard" percentage of devaluation per year foor high end speakers?

I think you also have to look at value from the buyers perspective:

1.  How much shipping am I going to have to pay
2.  What has been the average time a similar loudspeaker has taken to resell...as an example on audigon
3.  How well known and reviewed is the loudspeaker
4.  Do I have a couple of equally interesting alternative loudspeakers that I'm interested in
5.  How much do I trust the seller

These are just a few...therefore, as a seller, you can easily see how your actual selling price might range from 40-70%.

jbl studio speakers hold best value decades. best investment. the same reason i purchased fj-cruiser.

Wow i will be up all night worrying about that.

+1 Elizabeth nailed it!

Four years ago I purchased a like new pair of Revel M106 for $1250.   Listed them on Craig’s for $1100,  sold for $1000.  Buyer was happy,  I was happy to get $1k, effectively “renting” them for $250.  

I bought my my amps for half of new at only 13 months old.  Unlikely I’ll sell those anytime soon but they will be worth close to what I paid for a few years if I ever did. 

I like buying used if it functions like it should but it has to be super clean or I’ll pass.  Like anything condition and demand dictate resale value.