There is a site called Keebswap which aims to be a used mechanical keyboard market to replace the subreddit r/mechmarket on Reddit (a formerly popular topic-based discussion platform that ... probably still exists but I'm not sure). Note, I haven't used either of these; I just know they exist.

From my experience, used boards are mostly sold in the respective semi-local communities on Discord. The continent you live on is a good first indicator for what communities to join :)

Craigslist and r/mechmarket have been my go-tos to finding used boards. It's not very reliable as they pop up once in a blue moon, but every so often the stars align and I check mechmarket on a day when a board I want is there.

What's your budget? I have also been looking out for something similar, mostly on r/mechmarket (I have an ergo travel that I am trying to get to work consistently).

There are sometimes deals like this: (not sure if this one is still available) at $80 https://www.reddit.com/r/mechmarket/comments/162uc4s/uscah_dilemma_v1_choc_pink_w_paypal/

Another place I've found really good prices is the bastardkb discord, but like r/mechmarket the postings for split keyboards are infrequent.

If you have switches and keycaps from a donor mech kb, this could be a good option at $128: https://www.reddit.com/r/mechmarket/comments/162uc4s/uscah_dilemma_v1_choc_pink_w_paypal/

Edit for completeness: $120 one piece option just without keys/keycaps: https://ergohaven.xyz/shop/tproduct/339856256-880659810681-planeta

Not sure about others, I tried to sell me keyboard but without success, maybe split keyboards are a bit too niche and no one near me (HK) is interested. I just disassembled the keyboars to scrap the components for the next build, which is a bit sad considering a significant amount of time is spent on assembling it.

This isn't a full answer to your question, but as for where secondhand keyboards are bought/sold, I bought one from eBay a while back.