I booked a condo for three months and transferred the owner a deposit of 3000 THB. As a receipt he send me a handwritten receipt with all the correct details.

It's a legal receipt to keep?

Thank you

3000 Baht seems a bit low as a deposit anyways, usually they would take at least a month ahead of time. But yes, that seems to be the Thai style sometimes!

Did you view the place in person and meet them?

Before computers became common, everything was handwritten, so yes.