Beautiful rig.

Now you going to sell the baby d seats?
I am thinking so.
What do you think a fair price would be?I don't know. If twas me, I'd get back to Russell and ask what they cost new, and when built, search for comparable used prices for 3-peice Russell setups, create an add with nice pics of them on your trike, listing the build date and original price. Start by asking about $150-250 more than you're willing to take as a bottom-line, put them out on Craigslist and elsewhere, and wing it.
In my experience, you have a lot better chance of a sale asking high, and dealing down, than starting low and saying FIRM. Even though you end up with the same price, people everywhere feel better when you deal your price down. It's a psychological thing.
Twas me, I'd first list them here on VRCC classified only, at my rock bottom lowest acceptable price, as a favor to fellow members, and if no sale, then move to the world.
FWIW, I am thinking that the tan seats would have a better chance of selling in the hot and sunny South than the rest of the country.
I'd also ask Russell if they'd buy them, they probably won't, but no harm in asking.