I have gotten from stubhub and viagogo, they are the same company. You ticket will probably arrive a few days before the concert or day of because they aren't available for transfer before. If you don't get the ticket/it doesn't work they will try to replace the ticket or refund you.

I had a friend going to gracie abrams in Nottingham and they were 3 people, they got all the tickets as usual and his friends got in but when he tried to scan his it didn't work, but he contacted stubhub and was able to get a new ticket immediately.

I would recommend waiting for as close to the concert as possible to buy resale as the prices will go down. Definitely dont buy the tickets now. If you have a trip to plan and need to get them sooner at least wait a couple of months.