I want to sell vbucks on fortnite, in order to give them their vbucks i have to have access to their xbox login info to be able to purchase this. They could use an alt account if they want though, but is this allowed?
Requiring buyer’s account in order for buyer to get the product is not allowed.
As @Sparkling_Juice informed, requesting account information is not allowed and doing so could lead your account to a permanent suspension.
Keep in mind that Fortnite has a Gifting function which you are able to send items from the store to another user. Currently, this is the only available option.