Thiel pricing

https://www.hifishark.com/model/thiel-cs-3-7

Click on "Sold/Expired Listings" tab.

Check out Hifishark, as mentioned above, and also the completed listings on Ebay.

Audiogon blue book?

OP-

$5K would be a good start, should move quickly a this price.
You could set it higher and wait out the potential sale.

Thiel 3.7 is very popular speaker so chance to get higher dollar on auction listing is there as well starting from let's say $2k especially if feedback is large and solid. Otherwise I'd try to sell Craigslist first locally. It's always better since you don't loose much on transactions which can be BIG savings.
Let's say the local sale for $4500 can bring you same money as selling for $5k on ebay simply because no shipping and fees are involved. Obviously you may post an add for the higher dollar let's say $5500, but potentially you can go much lower than via ebay or any other selling sites.
I'd stay away from Audiogon, because I see too many amateur errors there suitable to most of beginners. Can't rely on security run by amateurs