Thanks for the reply achow101!

Knowing the receiving address is trivial since I would display the associated QR Code on my website.

I guess the issue is to know the transaction id when a sell is made, so I can validate it!

Should I, for example, ask the buyer to make the payment using his favorite wallet, then copy/paste the transaction id on my website so I can validate it using the blockchain?

Is this something websites/apps do? Ask the buyer to provide the transaction id?

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(I juste read your last paragraph) What kind of "message" are you talking about? A message on my website? A message somewhere on the blockchain?