The way I think it works(though I'm not 100% sure):

1. Buyer buys an item, sends bitcoin to the address handled and displayed by Selly/Shoppy
2. Once Selly/Shoppy receives the coins, the coins automatically get sent to your provided address minus the cutoff percentage

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Did I answer your question? Or are you asking literally how to do it technically? Because I'd suggest hiring a software engineer for that.