I'm also interested in listing packages of domains for sale. On DAN (the only platform I've used so far), I've resolved this by listing only one domain in the package for sale, and indicating the other domains in the description text field for that one package.
I was wondering whether it was possible to automate the grouping of domains in a package rather than relying on buyers and DAN to notice the description, and by coincidence I put this question to DAN's support yesterday:
"I am offering some domains as a package of multiple domains, usually spelling variants of the same name (e.g. with and without hyphens). Is there a way of automating the sale of multiple domains?"
Here was the reply:
"You can add in one of the domain's descriptions that you're selling them as a package and we'll transfer all the mentioned ones in the same transaction to your buyer." In other words, what I've been doing so far–although such a sale hasn't yet happened, so I can't confirm whether it works.