Every city has sellers who want to get rid of something quickly. You can afford to be patient.
Every city has people who need to get rid of things quickly: they are moving house, have just bought something new or are clearing out the home of a relative who has died. They price low to sell fast. You can afford to wait for a better price.
Pick one category you can judge, such as bikes, tools, cameras or consoles. Check what similar items actually sold for, not just what people are asking. On eBay you can filter for sold items. A good buy is something you can resell for clearly more than you paid, after costs, fuel and your own time.
Test every item before you pay, photograph it in daylight, put the model number in the title and answer messages the same day. Write down what you paid, what your costs were and what you got for it. Then you know your real margin.
This takes money and time. Your money is tied up until the item sells, and some items sit for weeks. Start with two or three purchases you would keep yourself if you had to.
