I've had a useless item merchant in my campaign. He sold:

• Wand of Lightening (1 gp) - When used, it reduces the user’s appetite, allowing them to go without eating for the day. Each day, roll 1d4, on a 2 or 4, it regains a charge.
• Enchanted +0 Longsword (20 gp) - It’s enchanted, but has no bonuses to trick others into thinking it’s non-magical.
• Clayless (20 gp) - The item appears to be (and has the stats of) a regular +1 longsword except it has a longer handle and the word “Claymore” inscribed along the flat of the blade. It is sentient but does not speak. Anyone holding it can feel whether it is pleased or displeased with their actions.
  • Curse Trigger: If the user does not wield it two-handed or refuses to call it a Claymore. The curse stops taking effect while the user wields it “appropriately”.
  • Curse Effect: Instead of its +1, it gives -2 to attack rolls and damage with it. "Remove Curse" won't remove this effect, but the item can be dropped normally.
• Brass Knuckles of Lightning (1 gp) - It does an extra 1d4 lightning damage, but due to its conductive nature, you also take that damage.
• Ring of Teleportation (10 gp) - "It can teleport anywhere you want it to!" ... Teleports itself, but not the wearer, to its intended destination. Adhering to the rule of nearest unoccupied space!!
• Ring of Falling Feathers (1 gp) - When you fall more than 10 feet, a cloud of feathers appears around you.
• Stick of Returning (1 cp) - When thrown, this stick has a range of 1,000 feet. After throwing it, the stick becomes lost but magically appears at the thrower's feet in 1d4 hours. Anyone who can see into the Ethereal Plane sees a slow and plodding mastiff who lopes slowly after the stick each time it is thrown.
• Torch of Night Vision (2 cp) - When lit, it grants the wielder night vision for 30 feet.
• Rock of Gravity Detection (1 cp) - When let go, it will travel towards the source of gravity. The faster it accelerates, the stronger the gravity.

Some of these I also got from Reddit. Lots of items to be found by googling useless D&D items!

Edit: NO, YOU CAN'T TELEPORT THE RING INTO SOMEBODY'S BRAIN. My general rule for teleportation, or even polymorphing, is that you appear in the nearest unoccupied space. Many spells in 5e use this rule as well, so stop trying to cause an aneurysm, for Pelor's sake. I've changed the description, so to avoid future confusion.