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For example, when I company I worked for sent me on a business trip, I was supposed to buy the flight ticket using my own money, then bring them then receipt, and then they would pay me the money back. Why couldn't they buy the flight ticket themselves instead? They knew when and where I am traveling; they were the ones deciding that. It would be easier for one HR person to buy 5 tickets at once whenever one of the teams went to a business trip, than for 5 developers to buy the tickets individually and figure out the rules.
This totally felt like a scam (use your own money to buy something we want to buy, and you only have our word that will get the money later back), except it didn't make sense, because it was a large international company employing thousands of people, and I have already worked there for months. And yes, it was perfectly legit. Just needlessly complicated and weird.
So the fact that a company does something weird and needlessly complicated doesn't necessarily make it a scam. It's just likely a scam when they do it before paying you the first salary.