Yes I've set up an ad and made pretty strict ID verification. I'm wondering if the fact that I'm selling on that site and then buying the same amount from another site with the hopes of making a profit is legal or not. Anyone have any experience with this? Or know about financial law at all?
If the trading of btc is legal in your country, then you don't have anything to worry what you intend doing is merely someone who is into buying and selling. Just like having a store for groceries by buying from an wholesaler to sell to a final consumer. All that you need to do is do it right by filing your returns and paying the due tax after carefully studying the tax law regarding crypto in your country with that, you are fine.
However, you just need to be more careful by taking time to know the business you want to venture into because as a new person going in that line of trade, you are target of some people who are always bent on who they want to scam.
