The way magma works is you basically put your offer out there, and shoppers that need inbound can choose you. After they choose you, you upload an invoice. The Amboss team then gives their own invoice to the buyer, they pay that invoice. Then you open the channel, once the channel is open Amboss then pays your invoice. They monitor the channel ID so you do not break the contract (for example going over a fee rate you promised not to, or closing the channel early). If you break your argreement you will get dinged on amboss reputation and people will less likely buy from you in the future. It's a fun way to get sats on the side if you're into lightning nodes.