Upon talking to a buddy of mine, the one who painted the Geeps, he thought my prices were "a little high" based on what he could see on ebay, and that "The ebay prices are generally less than what is advertised on the internet."
So, I've lowered the prices on the Geeps. See below.
I checked on ebay and couldn't find any S-4's, but the S-2's are going for ~$30, so the S-4's are not even worth selling. I paid Red Caboose (NYC) $120 for #8434 on 04/20/1995, and The Big, Little Railroad Shop (Jeff Kucsma) $80 for 8431 on05/29/1995. At the time, they were the only two I could find, anywhere. 30 cents on the dollar for excellent engines is ridiculous.