Have I been scammed?

Hi Reddit,

Throwaway account as to not embarrass myself. I've recently sold a domain to a person, and I think I've been scammed. Please let me know if this isn't the right subreddit to post.

I originally listed the domain on flippa - after a while, a person contacted me saying he was interested in buying the domain name. However, he could only pay in bitcoin, and not use flippa's system. So I created a coinbase address; but then he said he couldn't transfer to coinbase, but could only use this site: https://swooden.com.

So I created an account on swooden (it didn't require any personal details) - and he transferred the coins to the my address on swooden.com - Great! I transferred him the domain, and all was good.

Now I want to transfer the coins from swooden to coinbase - however, when trying that the site says that "Failed to approve withdrawal request. You can withdraw your funds only on address, which is registered and verified with you account. To verify you address with your account you need to make a deposit from this address. Minimal amount of the deposit is 0.02 BTC."

This seems very suspicious, and I don't want to transfer coins across to the swooden site (I've only lost a domain at this point, i don't want to loose more).

It should also be noted that when I look up my Swooden BTC wallet, there hasn't been any transactions on the ledger (even though the buyer has sent me coins to the address, and I can see the coins in swooden.com)

I've done a bit of googling and swooden doesn't come up at all; looks like the site used to be for a Swedish startup with wooden toys for all children