You can sell for whoever you like, but responsibility lies solely with you.
To that end, I would say don't divulge that you're an intermediary unless asked a question you cannot immediately answer, for one it doesn't inspire confidence in a seller or item, it plants the seed of doubt regarding responsibility should the item, for example an aeg, turn out to be a lemon.
That in turn opens up another can of worms, if someone sells on behalf of a third party, deals done & moneys passed to the owner, & then the aeg implodes on its first outing a week later, is the 3rd party gonna return the funds ?.
Its a bit of a minefield?