Not sure about buying, as only ever sold on there.
But recently sold two Foo Fighters tickets that were purchased on ticketmaster.
I listed them for FV including fees and you then get options as to how you will send the ticket to the buyer... are they mobile tickets or email etc that you can send across.
In my case the tickets were only valid in the ticketmaster app, but you get the option to transfer if you log into your account. So the buyer provided an email, and when I was notified they had sold on Twickets, I transferred the tickets to the email provided. I get a notification from ticketmaster that the tickets have been accepted and just wait for Twickets to send me the money. They say approx 7 business days after event I think, and got the money yesterday so technically 6 business days after.
You just put your bank details in when you create a Twickets account and money is sent there.
I have sold Peter Kay, Hans Zimmer and Foo Fighters this year, around £400 worth and no issues.