local craigslist is always a good option too.
I can tell you that the RX-V557 is probably worth at most $100-125 or so. Lower end, non HDMI receivers do not have much value.
With older speakers, you can probably assume it's worth about 25-40% of the original value, depending on how good the brand reputation is as well as the condition of the speakers (obviously). You can probably get better return on something like Paradigm, for example, than a random generic speaker brand with no name recognition. I don't know a thing about "Pinnacle" speakers so I can't say much more than that!
Also, high-end models will hold value better than entry-level models. The entry-level model from 10 years ago is probably worth only 20% of original value, whereas a really fancy high-end speaker from 10 years ago will be worth much more percentage-wise.