Selling Bitcoin for cash in my store
So far as I know, most store owners just rent floor space to a company that operates the Bitcoin "ATM". The store owner gets fixed monthly rental income and doesn't have to get involved in Bitcoin transactions themselves.

You don't need to invest in the "ATM". Just rent some floor space with power. I would expect the ATM operator (not the store owner) to arrange for Internet connectivity, but that's probably just a business negotiation.

One way to find a local Bitcoin kiosk operator is to search the map at https://coinatmradar.com/ - the operator's name is shown at the top when you click on a kiosk location after zooming in to an individual kiosk location.



As Wikipedia notes:

"Bitcoin machines are not ATMs in the traditional sense". A better phrase might be "Bitcoin kiosk" or "Cryptocurrency kiosk" since they generally deal with one or two altcoins too.

"According to Coin ATM Radar, there are more than 800 Bitcoin ATMs in the United States as of 2017, with small shop owners earning a reported $300 a month for rental space." (my emphasis)