I used to make $1K a month net profit selling on eBay. That was my goal... $1000 for fun/gun money, and I usually hit it. The trick for me was to sell stuff I could fit in a USPS Flat Rate box. That way you know what your shipping costs are going to be. I also kept my shipping low because buyers respect that. I avoid sellers that appear to be trying to scam me on the shipping. The post Office will deliver a stack of boxes to your door free, you print the shipping label from Paypal and schedule a pickup from the USPS website. I didn't have to leave the house to ship anything.
For me, I had to keep it simple to do it. When you start having to do a lot of packaging and running to the post office/UPS is when it gets tedious. Eventually, my product source dried up and I stopped doing it.
That was my game, anyway. It was fun though, good luck!