Afternic accepts is the key thing there. The terms put the onus on the seller to be able to deliver the domain not us to check if you can.
The terms are big and there are a couple places that seem to speak directly to this. I am not a lawyer and I cant interpret legal terms for you without getting myself in trouble but this part seems to me to be the clearest in regards to your question, quoted below:
Seller makes the following representations and warranties to Afternic:
- Seller represents and warrants that it is in fact the legal registrant and owner of each of its Listed Names, each free and clear of any encumbrances, liens of any nature, including third party intellectual property claims or registration fees due the Transferring Registrar (other than those created by the applicable registration agreement) ("Liens");
- Seller hereby authorizes and legally empowers Afternic to offer each Listed Name for sale via Fast Transfer, and upon completion of the sale, seller shall pass good and marketable title to buyer free and clear of any Liens;
- each Sold Domain shall be immediately transferable from the Transferring Registrar, without restriction; and
- Seller has no actual knowledge that any Listed Domain is the subject of an intellectual property claim of any third party.
I bolded the subsection 3. You as the seller warrant to us that you can immediately transfer the domain without restriction.
There are probably ways we could check and assume a lock on a domain but our assumption wouldnt always be right. Part of listing a domain for sale on our platform, or any platform I can think of is the ability to give the domain to the buyer now, not in a couple months.
As I said above we will try and save the sale because it is in everyone's best interest, the buyer, the seller, us. But really it is a bad experience and goes against the stated terms.