You wait until you are paid (most buyers will use PayPal .. you must have a PP account (otherwise you can't sell on eBay), and you have to check by logging into you account NOT by reading or clicking on some fake 'phishing' email that says 'PayPal Payment received' ..
After you have the money, you post using a 'signed for' service (not signed for = no proof of delivery and PP WILL refund the buyer as soon as they complain .. some buyers 'try it on' every-time they get a non-signed for package ..)
Finally, Vouchers are a total pain = you will have to photo-copy every one before you send it out becasue some buyers will let it go 'out of date' and then try to claim it was expired before you sent it ..
Finally, I suggest you read up on ebay rules and PayPal 'horror stories' before getting into selling .. it's a minefield, especially for the 'newbies' (the fraudsters and con-merchants will pick you out by your low feedback and try to take you to the cleaners - expect to get a lot of stupid bids from '0 feedback' crooks = they can bid-up the prices because they are either using stolen PP account or have every intention of reclaiming the payment as soon as the goods arrive) ..
Be especially aware of the 'can you send it to my 'new', , ''friends' , 'gift' address ? .. ALWAYS send to the address 'registered' at PP .. if you send elsewhere they will complain of non-delivery & PP WILL refund them ..