Probably the most popular way of making money on the side, re-selling is easily accessible and quick to start. It can be as simple as selling your own unwanted things on eBay. But it can also be more specialised, and more profitable, if you have a particular knowledge base.
Are you an expert on vintage clothing? Do you know how to identify a valuable collectible at a car boot sale? Carve out a niche for yourself, and your side hustle is more likely to soar.