Well I just looked around in my house for stuff I haven't touched for 3 months or so. I just looked it up on ebay and craigslist and posted it there. that was how I did my first 300€ bucks now. And then for flipping (buying and reselling higher), look for thrift stores, garage sales, yard sales and so on. If you want to know what to buy look for some youtube channels e.g.: Ralliroots, 10konthebay, rockstarflipper

Just one tip for eBay: don't do auctions. You will usually get low-balled. research the market price and price the item accordingly.