Call or go to your county's auditor or consumer affairs office and obtain your vendor license. The cost can vary depending on the area you live in.
Sign a contract with up-and-coming venues that have concerts on at least a weekly basis. You can inquire to bigger and more well-established venues, however, they will likely have a set group of vendors they have worked with for years.
Offer unique and trendy items that will set you apart from the other venues at the rock concert. Artists and venues will already offer merchandise such as T-shirts and music, so it is advised to stay away from that market. The same can be said for food and drinks, as the venue will likely have an in-house cafe or bar.
Use colorful or attractive signage such as neon lighting to catch the concert-goer's eye. In a loud, chaotic environment, such as rock concert, this is a necessity.
An electronic register with a built-in calculator would be best. But if space is limited a cash box with lock and separate calculator will be sufficient.
Always have plenty of small bills and change on hand to make change for customers.
If you intend to be a vendor at multiple venues, then easily portable tables and equipment will be needed.
Depending on what items you sell, a truck or van may be needed to transport your goods to and from concerts.
Storage spaces would be optimal for keeping your equipment and merchandise while ensuring it's protected from theft.