There's paperwork to be done besides swiping a credit card through a mobile reader. It would be nice if you could just hand the rider (or whatever athlete you're shooting) a card and trust them order the final print. That's probably not the way to maximize sales. You're at "an event". Things get lost. If they don't know where they put your card, whatcha gonna do when they can't come for you? Go to them. They've given you a minimal "down payment" to do the work. You gotten them to sign some documentation (including a model release) so you can identify who you have been contracted by to do the work. Once the paperwork is signed, take a shot of them holding the signed paperwork and showing their rider number so you can match up who and what. Once the image is ready, send the person an email with the URL to the website where they can buy a print. I'd limit the sizes available. You wouldn't want to put some work into an image and then have someone buy an 8 x 10. You're making "artwork" to be hung on a wall, so make it big enough to be hung. Start at a 16 x 20 and go up from there.
One nice thing about going with something like SmugMug is that they offer ancillary items. Maybe an add on sale could be a mug or a mousepad. There's no sense to being one and done. Another recommendation might be not to make it an obsession. If there's a sport you like and follow, why not make a couple bucks on it?