Books are an easy way to start selling. It is very low barrier and little knowledge is required. You can go to yard sales, thrift stores, and library sales to find books to resell.
Many people want to get rid of their furniture as it is taking up a lot of space, meaning that you can pick up many items for free. That means there is a great way to make decent profit from the start.
Bikes can be great for buying and reselling. There are always a lot of people searching for a decent bike, so if you find one you can be sure to sell it for a good price.