This website ask me to enter the address FROM where I would be sending BTCs. But every time a send some BTC it seems to be sent out from a different address... Should I use the one I always see when I say I want to "receive" some BTC? Is it my "main" address?
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1It sounds like the website you're interacting with doesn't understand how Bitcoin works. – Greg Hewgill Oct 3 '17 at 19:45
There is no from
address in Bitcoin.
https://en.bitcoin.it/wiki/From_address
Each transaction reference an unspent output (UTXO)
that was sent to you in a previous transaction.
About the address, you need to generate a new one for each payment: https://en.bitcoin.it/wiki/Address