It can happen with any market place: Afternic, Sedo, GD, etc...
The owner listed everywhere and sold in one place but forgot to remove the listing.
Sedo has
up front MANDATORY verification in place, and the problem seems to be far less of an issue there--at least for the initial listings. Afternic/GD/Dan do not require verification up front for the initial listings.
You are really spot on about the concern about some listings that were forgotten and not removed. Sedo's verification up front, for example, will not avoid the issue of domains that have expired or been sold since listing. It remains unclear whether they do this as well. For all marketplaces, it probably is a good idea.
My experience has been that my domains were i
ntentionally being listed by others, without permission and clearly without any verifcation by DAN or Afternic. They were longstanding ownerships by me The listings were clearly fraudsters trying to sell domains as if they owned them. In fact, my domains listed by one account would be removed, and then the same names reappear under another account.
When the fraudster tries to sell the domain, the sale fails unless they can quickly buy it from the actual owner (presumably at a lower price) and then do the transfer. This is called domain "front running," and apparently can work if the actual owner is using Fast Transfer or Sedo MLS premium transfer service.
Verification that the domains are actually owned by those listing them is important, and not just at the time of initial listing but also especially before a sale. It's sad that you had to go through this additional hassle.
Domains listed by others who do not actually own them is a widespread, well-known, recurrent problem. And there is no signs that it is going to be fixed any time soon.