What normally happens when you list domains at Sedo, that are already listed under somebody else's account, is that Sedo will review the ownership and then link them to your account. They might ask for confirmation or screenshots etc.
happened to me a couple of times. either sedo will contact you for confirmation or, like what i did, initiated an email to sedo and they'll likely assist you with that. best of luck on those new names
But if you're parking the domains with sedo, they'll ask you to propogate your nameservers to sedo's parking, won't they be able to tell from there who the real owner is?.
It happens all the time. Domains get moved from owner to owner, and registrar to registrar. Just send a note to customer service and they'll clear it up quickly. Just tell them to do a manual whois check and all should be good.