I think this would be a cool side gig and I have some ideas about how to make Bitcoin more accessible to the general public. But I don't want to break the law either. These would be normal flea markets where vendors pay $20 for a booth and then sell their wares.
Is there an authority I can ask? Do I need a special license?
Since sales are in dollar and the OP does not specify a jurisdiction, I will assume Washington state US. You may need a business license, depending on the municipality. There are various permits required for sale of certain wares, such as food, explosives or poisons, but otherwise no special municipal permits are required. A state money transmitter license is required, and there are a number of bond and auditing requirements that also have to be obeyed. The state has a website which implies the pertinent restrictions, but does not actually explain them or how to comply: you would need to hire a lawyer to determine what you have to to to be legal.
Those restrictions are not limited to flea markets, they regulate all bitcoin sales in the state, including internet and yard sale.
If you exchange bitcoin for currency (rather than accept it for purchases), you may be required to register as a Money Services Business (and follow the corresponding regulations). I recommend reading FinCEN's 2019 guidance document before operating such a business.
I'll note that FinCEN does enforce these laws, though small-scale offline operators aren't necessarily a priority; FinCEN is more apt to go after businesses that facilitate money laundering (e.g., by operating at scale or by running a mixing service).
Per https://www.fincen.gov/money-services-business-definition , these regulations apply if you have:
An activity threshold of greater than $1,000 per person per day in one or more transactions
So, a low volume flea market business may not fall under these regulations.
It is highly dependent on the specifics, but this US Federal case in the state of Michigan may give you some ideas of what exactly not to do:
https://www.justice.gov/Usao-wdmi/pr/2021_1018_Boden
TL;DR: selling Bitcoin to "customers" for cash, "customers" turn out to be drug dealers.
As the OP specifies the UK. Firstly Bitcoin has a cooldown time on transactions per time, so Bitcoin Cash may be more suitable (allows more transactions). Street markets often need a license, markets in somewhere like a private field usually don't unless there are structures that need built or traffic is affected etc. When selling Bitcoin you may have to pay capital gains tax on the profit. I know there are some machines across Europe & Africa where you can buy & sell bitcoin but I haven't seen any in the UK, including London. So perhaps there is a specific crypto law. Also if you buy/sell Bitcoin in in USD rather than GBP, all digital transactions using USD at ANY stage in the process have to get recorded with the US government. Even buying any currency with GBP, where banks use USD as an intermediate currency ( which they often do for less major currencies) have to do this.
