Brands that have the least depreciation

Or: which brands will you not want to sell for a very long time? Economically the best strategy is to buy once, and then enjoy for a long time. In audio real technological change is only slow. The only two quantum leaps that I can think of were the introduction of the LP record and the introduction of the CD and later other digital derivatives.

Pass Labs, Transparent Audio Cable...companies that are high quality and don't change models very often...Aerial 7b, 8b, 10T are terrific speakers whose value hasn't changed in many years, in fact seems to have risen lately...lots of cables if bought used will hold full value for many years

Simaudio Moon amps and phono stages has pretty good resale value

That is impossible to answer objectively

setting aside all the inevitable and highly variable and totally subjective and biased personal value judgement suggestions, it is analogous to asking..

Q "How long is a piece of string ?"
A "it depends "

in in this case it depends on timing, location, and other esoteric factors such as emotional attachment or its converse, and economic influences and technology obsolescence

For example, when the next inevitable / looming recession finally hits with a vengeance, the current market values of all will decline appreciably. Some will even crater, independent of any other affection for the unit.

A second example is the ever changing and accelerating technology advancements . There are plenty of quality build high-end brand products in excess of $10K when new , that have a market value of a small percentage of new within 2 years. Let’s remember that a 50% drop in the first year of most product is quite common , full stop..... INCLUDING 2-channel. The litmus test is see what you can actually get on a dealer trade-in or compare against the "solds" in audio forum ads.

Be prepared for a sobering gulp even in 2-channel ...just like in luxury cars obsolescence.

AV is a subset herein and the worst subset by far that needs little discussion intuitively. For example, many AVRs and processors will not be even be taken in on a dealer trade-in and are little better than boat anchors in the resale market.

Simply put, anybody who :

(a) contemplates buying gear driven by a thinking they can successfully hedge against depreciation in an ultimate contingent and variable future price arena; then

(b) they are unnessarily shortchanging themselves for best available performance now, and

(c) myopic to a fault and still being exposed to a rude wake-up call disappointment -- and likely shock-- if the current equipment line-up is line for change....

full stop.