The scam almost never looks like a scam. It looks like a friendly landlord who happens to be abroad, a flat priced just under everything else, and a message asking you to hold it with a deposit before someone else takes it. In Spain the mechanics are well known, and the checks that stop most of them cost one careful afternoon.
The classic version is a copied listing. Someone lifts the photos and the text from a genuine advert on Idealista or Fotocasa, drops the price a little, and reposts it. You message, you get a warm reply, and you are told the owner lives abroad and cannot show you the place right now. A relative will hand over the keys once you pay. The pressure is always the same: a good flat goes in days, so send the reserva today, by transfer, Bizum or Western Union, to a name that is not the one on the listing.
A second version hides behind a "seasonal" contract, or a sublet dressed up as the real thing. Someone rents a flat short term, advertises it as a long-term home, and takes a deposit from two or three families in the same week. The address is real. The person is not the owner. By the time the keys fail to appear, the phone number is gone.
Most of these cases share the same handful of signs. Any one of them can be innocent. Two or three together is a reason to stop and slow down.
None of this makes a listing fake by itself. Plenty of owners genuinely live abroad and rent through a local agency, and plenty of good flats go in a weekend. The difference is paperwork and patience. A real landlord can prove who they are, and does not fall apart when you ask.
Before you send anything, do the boring things.
If the owner is abroad and works through an agency, all of the above applies to the agency instead. A serious one will send you a draft contract, its CIF and the owner's authorisation before it asks you for a euro.
When you are ready, pay in a way that leaves a trail and matches the paperwork. A transfer from your bank to the account named in the contract, or to the agency's company account, is normal. Cash to a stranger is not, and neither is a transfer to an individual's personal account when the contract is in a company's name.
Understand what each payment covers. In a normal long-term let in Spain you typically pay a reserva to take the home off the market, then the first month and the legal deposit at signing. The fianza for housing is one month, and in Catalonia it has to be lodged with INCASÒL rather than kept in the landlord's drawer. Extra guarantees can be agreed, and often are for expats without Spanish payroll, but they belong in the written contract, not in a chat. A landlord who refuses to put the deposit on paper, or to register it, is telling you something.
Act fast. Report it to the Policía Nacional or the Mossos d'Esquadra and keep every message, receipt and account number, because the bank and the police both need them. Your own bank may be able to recall a transfer if you move within hours rather than weeks. Consumer offices (OMIC) in your town can advise, and the platforms will usually pull a listing once you flag it with evidence. Getting the money back is not guaranteed. Not sending it in the first place is the only reliable defence, and it costs one careful afternoon.