My in-laws booked a villa in Poland for the family vacation, and I found out too late they already paid half of the price by a bank transfer. Apart from the bank transfer itself I didn't find any red flags but nothing strongly supportive of the venue's legitimacy either. They have a website, are listed on Booking.com, and the only review on Google Maps is by the owner. I read in the comments here that payment by bank transfer is not unheard of in Europe when renting directly from a landlord. The owner has been corresponding with us, is responsive and has decent English. He sent us a rental agreement to sign but I have no idea if it would hold for us in a Polish court. I did not read it. Overall it does appear like this is just some family's country house which they figured they could now rent as rural Poland is gaining popularity among tourists.
Am I overreacting? My in-laws want to pay the full price in advance to get a lower fare but I am definitely against that. Do I have any more methods to verify this rental is legit?
Post-travel edit: The rental was genuine, the owner is a lovely person and we thoroughly enjoyed our vacation. By now the place has multiple reviews on Google Maps (including mine). Hopefully this makes it clear it is legitimate for future skeptics who read too many questions tagged 'scam' on SE sites.