Go to thrift stores and yard sales. Take your phone. Find items that look like a good deal and search them on eBay. On eBay go into advanced search options and make sure you check "completed items", this will show you what actually sold and at what price it sold. If the item you are looking at is $5 and goes for $50 you just made 10x your money. Buy me the item(s) and sell them either on eBay or Craigslist or FB sales groups. If you cross list the items on different platforms make sure to remove sold items asap.

This will allow you to turn $100 into $500 if you hustle. If you half ass it results may vary, but if you put your all into it this will work.