I have the chance to buy a used Stax SR 80 and a SRD 4 for about $175.
However, I'm a complete electrostat rookie and have no idea what things to look out for when buying one used. I already searched, but couldn't come across any typical problems outside of a damaged membrane.

Could the Stax owners among you perhaps provide me with some tips? I would very much appreciate it.

No tips at all?

=Be sure to clearly ask the seller if there are any volume imblances at all, between the left and right side ( this is irrepairable in an electret driver )
=As well if they are very faint  even when turned up loud, then also avoid them.
 
Both issues can happen becuase the electret diaphragms have a permanent embedded  charge, and often from overdriving them there is a parasitic charge that builds up and makes them permanantly lower in volume,  that and sometimes very poor storage conditions can affect this too.
 
but generally in ideal half decent storage and use, they should maintain a charge for longer than a person lives.
 
 
BTW Usually any issue is with the headphones themselves and not the adapter unit that you hook up to your receiver/amps speaker terminals, since all there is in there are transformers and/or a few capacitors, usually very simple in there.

Thanks for your great tips! Just what I wanted to know.

Best wishes for 2015!

Seems a bit pricey, though.

I had a listen today. Although the headphones looked to be in good shape, the left channel was not nearly as loud as the right channel. Needless to say, I didn't buy them.

Thanks again for the info. Otherwise I might have taken the gamble thinking it was the adapter unit.

Good to hear. A person can always transplant proper drivers into them, but buying the spare SR-40 to do so you might as well find another proper 80. If the 40 is matched that is