Transferring a ticket
Where the creator allows it, you can send a ticket to someone else from the ticket itself. It leaves your wallet and arrives in theirs, and they can present it independently.
When they accept, the ticket is given a new code. Yours stops working at that moment — including any screenshot or printout you already had — so there is never a point where two of you are holding something that scans.
Transferability is a per-ticket-type setting, so some tiers on an event may move while others do not. If the option is not there, that is the creator's decision rather than a fault.
Why not just send a screenshot
A ticket admits one person once. Send a screenshot and you both hold the same code — whoever reaches the scanner first gets in, and the other is turned away at the door with nothing to appeal to. A transfer cannot end that way, because accepting it replaces the code and only one of you has the new one.
Transferring properly is the only way both of you can be sure who the ticket belongs to.
Resale
Selling a ticket for more than face value is not allowed, on Tiqstar or anywhere else. Neither is selling platform tickets through a third-party site. Tickets found to have been touted can be cancelled, which leaves the buyer with nothing.
If you cannot go, transfer the ticket to someone who can, or check whether the event's refund policy covers you.
Buying from a stranger
A QR code sent by someone you do not know is worth exactly nothing until it is scanned, and by then you are at the door. If it has already been used, or was cancelled after they were refunded, you have paid for a picture. Buy from the event page or from the waitlist.