Right off the bat about reselling clocks online - be careful with shipping them. A glass cover is so common and easily breakable, I highly recommend a lot of bubble wrap, styrofoam, maybe pool noodles, and definitely a box within a box for shipping. Now having said that, this may not be a super exciting niche, but if you're out sourcing and see a cool clock, especially a couple of these BOLOs, definitely pick it up.
I've found a few clocks at the bins and actually kept one because our household is clock crazy - we have a clock in every rooom of the house, including bathrooms. I guess we just really want to know what time it is at all times.
Clocks can sometimes be pure decoration, too, so an interesting design or quirky design on a wall clock is worth the time to look up comps on. Brand new clocks will sell well and as far used, it's got to have something unique or desirable about it. Sold clocks on eBay right now are on average about $20 for a standard pre-owned/used one.
Brand new clocks are a good seller on eBay, as I just looked at sold clocks and as of today, brand new clocks are going for average $20-40. As an apparent "clock collector", finding a clock in the store is not as easy it as should be. Sometimes a standard boring black or brown clock is just not fitting the bill. And sometimes the face is hard to read, or you have to push a button to get it to light up - just give me a simple easy to read clock. Turn to the selling platforms and boom, there's a plethora of clocks to choose from.
Vintage clocks can be a very good flip. My "white whale" in the clock niche is an old school clock/clock radio with flipping numbers. These can resell for good money and will probably be a cheap buy since thrift stores will often think it's just an old clock with no real value. Ha! Antique clocks can fetch high dollars, at least $100-$200.
Clocks with character are good ones to grab to resell. Clocks with comic book characters, movie or TV themes, or just outside the standard black or brown are all good ones to pick up.
One last note about shipping clocks - if you can, on wall clocks, remove the glass and put painters top across it and wrap it separately. A buyer would rather reinstall the glass cover than receive a broken clock. If you can remove the glass, on either a wall or desk clock, still tape the glass with painters tape for some extra reinforcement.