Are We Allowed...

to sell WoW accounts here? Just curious.

Pretty sure no, because that violates ToS of Blizzard, which would be considered warez.

Or by eBay’s vocabulary… are we allowed to sell the time we spent building up our WoW accounts, simply including the account as a free bonus?

There are inherent risk involved in buyin a WOW account, One main is the banning of that account. Another is the original owner can at anytime recover that account using the original CD key and setup info.
So I would advise against it.