Hello everyone! I'm looking forward to selling two of my characters via char trade, but then only death knight will be left. Since it's not possible to have one unless there is another 55 lvl+ char, will I be able to log/play dk if that's the only character I have on the account?
Yes, you need a lvl 55+ only to create the DK.
That restriction applies to creating a DK, not playing it, so your existing DK will be just fine. And someone correct me if I'm wrong, but having premium status also lets you make a DK without the lv 55 character requirement, doesn't it? And since OP is selling characters, the account must be premium.