How to protect yourself as a seller from scam?

[art_vision_x3]

How to protect against people, who juggle multiple sellers and don't pay? I think I have been scammed, worked for free.

Do you start working on a project, without any active placed order or not? If buyer asks you and tells you what he needs, but won't place an order?

I could make some sample and ask payment for it, so I know it is a serious buyer. Should I ask payment, before the sample is delivered or after? Or just that buyer places an order and when I deliver he must pay for the sample so I protect myself against scam? When he pays and places an order for the main project I can start working on it?

I read quite some posts, but I still didn't find precise answer to that questions.

Best regards,

Alex