You could have a vending machine wired to a deactivated (one game start) sentry. When the vending machine’s bought, it activates the sentry, and it is placed on the player’s team. I think you can do that with the triggering player’s team block.
Here’s some more sentry guides that may help: https://forum.creative.gimkit.com/t/how-to-make-a-talking-and-trading-sentry/2250