Just to be clear, you've heard of departing tenants trying to rip off people who are desperate for somewhere to live and would like to whether it's ok to do the same?

You can certainly offer to sell your furniture to a new tenant but you can't force them to buy it and you certainly can't 'make it a condition of the lease' when you are not a participant in the contract. If you are the 'gatekeeper' to the applications going to the landlord you could filter out the ones who say they don't want the furniture, but it they are sent direct to the landlord you have no control. And even if someone says up front that they're really keen to buy it, once they have signed the contract they might change their mind. Suggest you make your asking price very reasonable to maximise your chances of a sale...

[FYI I've sold furniture to a subsequent tenant at reasonable prices and entirely optional, and I've been subject to pressure to buy second hand furniture at high prices from the previous occupant... although in that case he was the landlord so had all the control.]