Banking Scam!
If selling anything in Barcelona, people are making use of the very flawed banking system whereby whatever amount you input on a lodgement comes up as pending in the receivers account (so you could lodge an empty envelope, input 500 as the amount, and it will remain as pending in the receivers account till the banks check the next day). People are mostly using this when buying phones from people not aware of how the bank lodgements work. So they check their account, see an amount pending, and give the phone away on this basis.
Cian