You can sell it yourself. Unfortunately, most timeshares sell for 0-10% of what you paid the developer. You need to be prepared to take a huge loss or you will be unable to sell.
There are a lot of timeshare resale scams out there. You may get calls from someone claiming to have a buyer for your timeshare. These are scams and they will try to get you to give them money. They don't have a buyer and will not sell your timeshare.
There are also places online that will charge you hundreds of dollars to advertise your timeshare for sale. They will give the impression that you can sell for a lot of money if you list with them, but this is also a scam.