This isn't exactly passive but...

I used to buy old furniture, refurbish it and resell it on craigslist. I made generally about $20 - $30 an hour. I also kept things for myself sometimes and slowly replaced different furniture items with nicer and nicer items. One year, I had 4 different coffee tables in succession. I found a cool coffee table and kept it, then I found a cooler one weeks later so I kept that and sold my previous one. Repeat that 3 times.

I'd mostly troll a certain thrift shop for antique-ish and mid-century items. It doesn't take long perusing Pinterest and design blogs to get a bit of an eye for what people want to consider collectible. Plus often if you look at something and squint, you an see if shaping it up will make it into something people want. I occasionally hit some garage sales, estate sales were too high priced and competitive. Sometime going an hour before they closed, they would drop the prices super low. %75 off would be better for them for them than not selling AND having to move it to get rid of it. Not always though.

I mostly just used Old English oil or Minwax furniture wax on items. It wasn't unusual for me to buy a $20 dresser, spend an hour giving it a cleaning (with Windex or other dollar store cleaners) and an oiling and sell it for $80 - $125.

I would do some small carpentry repairs on items, usually broken drawers, loose legs and occasionally a crack in the piece that was on a non-essential or non-structural area. You learn to avoid the repairs that are time consuming, difficult to get right or especially those--this is hard to describe--that will either be super easy or will fail no matter what you try with no in between. I avoided painting things, too time consuming plus I hate covering good wood. If anything is missing a knob or matching decorative hardware--it was an instant "no". You'll never find one that matches just right without spending a ton of time--if at all--and if you replace them all, it's expensive. Plus new knobs usually look weird on an antique and ruin the aesthetic of a mid century item.

I have had dozens of people tell me that I am a magician because I could take an old dried out looking item and bring the finish back. Honestly, it was the oils and the wood itself. I's take a piece like a dresser, table or chair, and after cleaning it for 5 - 15 minutes, just spend another 20 wiping the oil on. You just go over every bit over and over to rub the oil in well and also that takes the excess off. Give it a 2 minute wipe down an hour later if you can, using the same rag but without adding oil. (sometimes the wood soaks the oil into itself, but then it's wet and starts to expend. Then the oil gets pushed back out and sits on the surface. This step gets rid of that.) Give it a super light coat of oil the next day, looking for areas you missed or that soaked up more or less oil than others. This takes maybe 5 minutes.

Dressers were my staple. Everyone has at least one dresser, people use them in other rooms for storage or decor. Chairs were okay deeding and also would sell in even number better. Coffee tables, especially mid century items went easily also. Headboards and foot-boards I would buy if they were exceptional in appearance or super, super cheap. With those, any physical repair needed would also deem them a "no".

All my supplies fit in a dollar store plastic basket that was about 14"x14"x12". A few bottles of surface cleaner, oil in dark medium and light and a can of wax. Then a bunch of rags, use the same rag for the oils, one for each color. and one for wax only. Some steel wool for scuffs or hard stains, a small hammer, a few screw drivers and a ton of different sized screws and nails, almost all in the smaller than average to tiny varieties. I very occasionally used a drill and do have a couple long bar clamps for gluing.

The skill needed to bring a finish back is minimal. I'd say if you can clean a bathroom well or polish a chrome bumper, you can make a $10 thrift store item look like a $150 one. Just rub the stuff on and it does the work. People will call you an artist and not realize they could do the same.

I sold on craigslist and usually the people came by in the evenings when I was just making dinner or getting stuff ready for the next day anyway. I have a covered porch so they'd never come into my house, I have a large dog with a scary bark, plus I had a few hidden aces up my sleeve in case anyone got cagey. No one ever did. Craigslist can be a gutter of society but the people wanted what I sold were about 20 somethings, soccer moms, "artsy neighborhood dwellers" of varying demographics or empty nesters. I had a 4 text limit and if someone texted me more times than that, I'd do my best not to arrange to meet with them. That sounds like a dick move, they just want to know about the item, right? But like clockwork, those were the people who would no-show, super low-ball me once they got here or reschedule 20 minutes before they were due. People got one strike and they were out.

The thrift store I went to to was near where I would run errands and so I would stop in there for 5 minutes every time I went to get groceries or whatever. I made one special trip a week on Saturday mornings, right when they opened. I was there 1 - 4 times a week, usually about 3. So it did not take much time out of my day or routine to get stuff. It came to a close because the one thrift store I used to get stuff from really fell off in their selection. In the Pinterest craze, more people started keeping things and "Up-cycling" them themselves. My store also lost a deal with some organization and suffered a drop in items. They also used to make regular pick-ups in areas and I think they just saturated it an sort of squeezed the sponge dry.