1st @Plebeian Market (easiest to start with): i). Use desktop for that : https://plebeian.market), you'll need a signer extension (try for example Nos2X or Flamingo ; 1st login with your Nostr 'profile' and then use it to get onto Plebeian ; btw. it will be useful for many other Nostr desktop platforms). ii). Through Dashboard start adding products/collection of products etc. - this part is super easy

@npub1yye4...hmd0 - reg. BTCPayServer - watch this (to understand the topic a bit better) + more directly on their website

BTCPay Server Walkthrough | BTCPay Server

BTCPay Server Official Documentation

; and forget PlayWithFlash - i see now they chnaged their business model (not worth it)