My 'side project' is pretty conventional, but it surprises people to learn how much I've earned from it with very little effort. At least once or twice a week, I use my lunch break to walk to the local thrift store. I look up items on eBay as I browse and buy anything that would net me $20+ (basically to cover my lunch). I stick to the 'hard goods' section (things like electronics, games, DVDs, home appliances, books, etc). I avoid clothing which is difficult to browse quickly, difficult to list and difficult to find comparable items. I also avoid anything that would be unwieldy to carry home or ship.

The first year I started doing this, I grossed about $20000. Last year was bad. Our thrift stores shut down because of Covid. In late June of this year, I started back up slowly because our thrift stores came back. My gross for the last six months is $3,238.63. And again, because of Covid, I've stopped going daily like I was which cut into my fun money.

Last year I bought a brand new pinball machine from my thrift store finds.

Some tips:

If you can go every day, you only have to look at the 'new' stuff which saves lots of time.

The eBay app has a barcode scanner so you can look up anything that's in its original packaging by just scanning it.

Don't pass up anything vintage looking that's still sealed (even if it looks worthless). I sold nearly $100 worth of new in package vintage incandescent bulbs a little while back. I got the lot of them for like $10.

Save all the boxes you have coming to your house in advance of starting this. You can't sell stuff that you can't easily box up and ship.

Never use the post office or UPS/FedEx store. You can get a significant discount through pirateship.com (free to use, offers steep US Postal and UPS discounts) or even eBay's own shipping.

EDIT: A few more tips...

Once you're shipping at least one thing a week, buy a label printer like a Rollo Thermal Printer. It will save you lots of wasted time and energy in printing out, cutting the shipping label to size, taping it to the package securely, etc.

Invest in a good industrial shipping tape gun (not those cheap little plastic ones), get good brand-name shipping tape and a couple of large rolls of bubble wrap... also, start saving the packaging that comes into your house from your online purchases to be reused.

Get a bunch of 'newsprint' packing paper for packing up your stuff.

Basically: don't cheap out on supplies.

Stay organized. I use a set of plastic bins in the basement to store my items once they're listed.