Generally I take pot luck up to about £30ish because normally you can sell things on easier if they are cheap.
You can normally tell with speakers etc. from how they look in the pictures, drivers, binding posts etc.
With amps and things there are lots of older brands (trio, sansui etc.) who made good stuff in their day and that does keep the price down a bit. Its the same with old goodmans speakers.
I generally stick with ebay as shops tend to put on a hefty markup but make sure you set a limit as most bidders wait until the last 5 seconds and you can find yourself paying over the odds.
I was lucky with the marantz because the title was poor and didn't mention the brand name so lots missed it.