Color me slightly wealthier.
Here's an easy one that has been raking in side-cash for me for years now... big lots of colored pencils. But not just any old brand of colored pencils. I specifically seek out, pick, and flip Prismacolor pencils.
I find these things all the time and can usually get them for 10 cents each or even less. They are normally sold in sets 12, 24, 36, 64 or 132 originally (and also individually) and can be quite costly to get new at an art supply store. But, alas, once a few of the pencils from the set are used up or lost, the rest sit around a desk drawer for years. Once the long, slow path towards perceived uselessness is complete, they are donated to a thrift store or left in a box at a garage sale or just thrown away.
After all, who would want to spend a couple quarters on eight or nine random colored pencils? You, that's who! These suckers cost about $1.50 each new, and folks will usually pay nearly a buck each for them on eBay, particularly if offered as a large lot.
And they don't even have to be unused to fetch the dollars. As long as they are long enough (say, over 6 inches), they are a solid pick.
After flipping (and occasionally coloring pictures) these things for years, I finally got a bit curious about why Prismacolor pencils are made by different brands. So here is a little history: Prismacolor started in 1938 by the Eagle company, then changed name to Berol in 1969. It was bought by Empire in 1986 and finally by Sanford in 1995. Sanford is currently owned by Newell Rubbermaid, but the current Prismacolors have the Sanford name on them. In the UK and Canada they kept the Berol name until recently, and they were also called Karismacolor (and they have oak colored barrels rather than solid colors).
Update: The day this post published, I came across Scholar Prismacolors! So look for those as well.
I have had no problems mixing Prismacolors of all brands into a single lot. In my experience, a vintage Prismacolor with the Eagle name does not sell any better or worse than the others, so there is no need to separate them into a different lot. I also will include doubles (and triples or more) of colors in my lot. They do not all need to be different colors to be sold together.
There are currently 150 colors offered in the Prismacolor line. This isn't really relevant to them being flipped, but I just thought that was an interesting tidbit to share.
You will also find other types of art supplies with the Prismacolor name, such as watercolor pencils and markers. While I have had more picks of the straight-up color pencils, I don't see why these others things won't do well in a large lot if you can snag them on the cheap. Just make sure to test those markers, because they will dry up on you.
What I haven't done well with, however, is flipping lots of other brands of colored pencils. So leave those Prang and Crayolas alone.
So don't be afraid to get dirty digging in the pencil drawer. If there are other art supplies or inexpensive items that are pretty easy to collect in lots that you have had success picking and flipping, let me know in the comments. Have fun out there.