My take on your use of the word "dependable" is the synonym "reliable" as in probability of failure. In this case yes, an amp that has been pushed hard will be less reliable. Fuses might be more likely to blow if they've experienced repeated heating to near their "blow" point. The output transistors might likewise have experienced some aging from approaching their thermal limits. The power transformer will likewise have experienced some aging from high temperature operation. The power supply capacitors might have aged some from a history of high current.
Though the answer is yes, an amp that has been run hard will be less reliable (more prone to failure), I would not expect a large difference in reliability between it and a "lightly" used amp. Well made amps just don't fail all that often. A heavily used amp might survive many years. On the other hand a lightly used amp can fail from any of many possible "factory defects" that remain latent until exposed by use.
All else being equal, and having a choice, the amp with a "light use" history would be the better choice and worth a few bucks more.
As for the seller's honesty or judgment as to what's hard usage and what's light usage, the best bet is always to know the seller. Give him a call and get a feel for his situation (how he's used the amp). Also be sure he's the only previous owner (he likely can't vouch for the previous owner(s)).
Though the answer is yes, an amp that has been run hard will be less reliable (more prone to failure), I would not expect a large difference in reliability between it and a "lightly" used amp. Well made amps just don't fail all that often. A heavily used amp might survive many years. On the other hand a lightly used amp can fail from any of many possible "factory defects" that remain latent until exposed by use.
All else being equal, and having a choice, the amp with a "light use" history would be the better choice and worth a few bucks more.
As for the seller's honesty or judgment as to what's hard usage and what's light usage, the best bet is always to know the seller. Give him a call and get a feel for his situation (how he's used the amp). Also be sure he's the only previous owner (he likely can't vouch for the previous owner(s)).