What to do with Knoll Platner Lounge Chair  

well if they are matching and one has a tag that says knoll then it pretty safe to presume that the other is knoll even with missing tags. They fall off, its a fact of life
with selling that is all up to you. dont look at 1st dibs to base prices off of as they are pretty inflated. do craigslist (free) , etsy (they take 3%) ebay (10% plus paypals 3.9% and 10 cent transaction fee) or the many other selling sites that are out there. (few sites are devoted to furniture)
if you do not do this sort of thing and dont know how and/or cant take the time to learn, take the easiest route and sell it to a dealer or to a collector where it benefits both you and them and let them take the time to sell, pickup, ship etc and just take the profits for the little time invested and be happy

Thank you so much! That is helpful. I think we will figure out the dealer route. We would be happy with just getting our money back ($55 for all 3)! How do we know if what the dealer is offering is a fair price? Thanks again!

You can research sales on Ebay for a fairly accurate picture of current market value. Do a search and then click on SOLD items on the left sidebar. It's not enough to look at just the completed listings since not all of those will have sold. If there aren't a lot of listings, then you can track them over a few months' time.
It might be better to consign them instead of selling outright to a dealer. Just be sure to get everything in writing! I know of a dealer who did a lot of business on handshakes alone and now he's gone and his inventory was auctioned off with no records to prove who owned what. His consignors lost their stuff. I think this is not that common an occurance but it does happen and you should make sure you're protected.

Thank you so much for all of your help! We really appreciate it!