There is a simple rule of thumb for timeshare offers:
If you have to pay ANYTHING upfront - no matter what they call it - it's a scam.
Reputable brokers take any fees out of the proceeds of the sale.
This "buyer" is going to wait until they have you completely hooked, and then they are going to come up with a bogus fee you have to pay, before you can get the big check.
Once they have your money, you will never hear from them again.
This is a scam we hear about every single day - if you fall for it, you will lose every cent you give them.