Even if you have your own stack for software, you still need someone for payments, and that's where it hits you.
Shopify was letting Kanye sells nazi merch, they don't give a crap for most things unless it's trademark issues.
Opensource is the only way to free sellers from propeitary SaaS and marketplace middlemen. I’m actually working on building an opensource Shopify for every vertical from restaurants to gyms to hotels.
There’s an adapter system and we have Stripe and PayPal adapters but some users have created one for Solana Pay and Coinbase. Essentially it’s up to the seller what payments they want to support. Create the adapter functions once and you’re good.