Find a problem they are experiencing and introduce them to a solution they can self-host to fix it. Expand from there.
I began my self-hosting journey 7ish years ago with media piracy and a desire to watch/access my files wherever I was. Learned of Plex, then Emby, Reverse Proxies, Domains, SSL, and on and on…
Today I’m running 24+ docker containers and some miscellaneous stuff, across 3 systems; that’s always accessible via my domain/vpn.
Coupons often drive sales for items that people usually wouldn’t buy themselves, sometimes even to people that normally wouldn’t shop there; then once they’ve tried something they haven’t had before, the customer is more likely to purchase that again in the future.
Then there’s additional items you add to the order beyond the coupon.
Finally there’s some rather insane profit margins on some of those items. Even with the coupon, they’re still making profit.