I would comb ebay for your best prices....use a reserve so you're not swindled by a low-baller drifting through looking to flip, if you're going to sell. When the STA-2100 was 'discovered' by the ' I wanna find a great receiver, but don't want to pay a lot' crowd some yrs ago, prices shot to the moon. The traditional Marantz and Pioneer crowd keep the values of the good receivers up & steady. The Sansui Quad receivers suddenly went gaga a few yrs ago.
If the RS receiver wasn't particularly notable like the STA-2100, they don't bring big values. The cassette deck value would be about nil....unless you can prove it's a major brand like Pioneer or Nak. The Optimus might fare the best of the lot and more if you can tie them to a major brand back in the day.
I can't really get excited about 70's stereo gear anymore, because they all need to be gone through for any kind of reliability factor.....even the speakers need new surrounds. Tape decks being the worst....ask me how I know...lol
Audiokarma, www.audiokarma.org , is an excellent site for anything having to do with music and audio equipment, especially vintage audio equipment. It is free to sign up and to join. It is a very friendly sight with some truely amazing people that all share a common interest. Be sure to post your question about value in the "Dollars and Sense" forum. It's great to see the Realistic name getting some long overdue credibility. Look me up when you get there if you care to. My username is the same.
By the way, if you are looking for information about repairing or restoring your equipment so that you can use and enjoy it again, Audiokarma is the place to start.
(No affiliation with www.audiokarma.org , just a member.)