There are some good sites over the internet but most are scams.
Personally, I would suggest you to use **** or seatwave.com. However, if you are using ****, you should wait until 45 days before the match so that you are able to open a dispuite if there nis a problem. On the other hand, there is no problem with seatwave. this site won't pay the seller until the event has took place. Actually, I am using it to sell some extra french open tickets I have.