What are some of the legal things to keep in mind before launching a product that revolves around cryptocurrency transactions?

Probably varies per jurisdiction so it might be wise to talk to a financial advisor / lawyer at some point.

If you're just starting out and it's about relatively low amounts I personally wouldn't worry about it yet. You'd be unlikely to be a target. Of course, proceed at your own risk.

You might want to keep in mind that a lot/many/most countries have decided that Bitcoins are securities. As such there might be laws you have to comply with based on your jurisdiction.