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?
1It sounds like the website you're interacting with doesn't understand how Bitcoin works.– Greg HewgillOct 3, 2017 at 19:45
There is no
from address in Bitcoin.
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