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

another variable is how often the manufacturer changes models, or changes a speaker without a model change designation...some 7-10 year old speakers are still current, some have been upgraded/changed a number of times...

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!