Sir Mix-a-Lot (unrelated) is a traveling potion salesman who shows up for my party at suspiciously specific times, and generally has discounted potions specifically tailored for whatever they happen to be doing at the time. For example, if they need to be really strong, he’ll have a bottle of Sir Flex-a-Lot’s Magical Muscle Maximizer, which does increase the strength of one’s muscles, but not of their bones or connective tissue (it was designed to be used only in bodybuilding competitions), so whenever the drinker does a STR check, they must also make a CON saving throw to avoid breaking a bone or tendon. Need to decipher an ancient text? Try Sir Scripts-a-Lot’s Polyglottal-in-a-bottle, which will let you read unknown languages, but also comprehend all unknown languages, even those of the plants and animals around you, making it very difficult to concentrate on any one thing. (inspired by https://www.smbc-comics.com/comic/springtime)

A potion of True Healing… heals 1d8+2 damage, recipient MUST truthfully answer the next question they are asked. Sell the characters 6, but don’t tell them about the truth serum. Let them figure it out on their own.

Boots of Elvenkind… except Elves can hear you.

A bag of holding that contains infinite clowns. Every time it is opened, 1d4 clowns come out. The clowns are useless in combat and attempt to distract, annoy and mock the holder. While this could be used as a distraction, the clowns will follow the holder, drawing attention to them. You could create a table for what kind of clowns you get (mime clowns, pie throwing clowns, balloon animal clowns, magician clowns, etc). The clowns will wander off after 1d6 minutes. Where the clowns go and what they are (Illusions? Demons?) is unknown.

Scroll of summon wisp.

When used nothing appears, but you gain a speech impediment for 1d6 days.

Arrows of Accuracy: These arrows will always hit. Starting from the target, then out from that point, the hit roll is checked against defenses.

If nobody is hit, the arrow strikes the firer.

I gifted my party a sapient dagger. It was really good too. It was a 1d6 but let you attack again, but rolling a 5 or less on the die makes you insane because the dagger insults the user so badly for missing. If it happened 12 times, the character died. They used it from level 3 or 4 through the end of the campaign at level 18, since there was no cap on how often you could attack again.

Not quite the same, but a Paladin in a campaign I was in once bought a Shield of Missile Attraction for cheap because the shopkeep thought it was cursed.

Wand of Wonder What This Does

A regular wand of wonder that comes with a gambling addiction.

Cube of instant castle: Say the keyword ‘open’ to transform this cube into a '200x’200 castle. The transformation happens instantly, and if you’re caught in the area of effect, be prepared to get smashed. The cube is hard of hearing.

Sweaty Sword:

Really good sword, strong steel. The handle is wrapped in living leather harvested from the palms of a cursed pervert. It’s always slightly warmer than your hand and it exudes a sticky substance that enhances grip. Smells like corn.

Survival Stew Balls:

A fried ball of…food. It’s rock hard, slightly too big to hold with one hand easily, completely impermeable, and covered in a flaky, delicate panko breaded crust. To eat, boil one in 5 gallons of water to produce a pot of stew. The flavor is different for every ball. Never cook two in the same pot at the same time. Wash the pot thoroughly within 6 hours after removing from heat. especially if it’s made of iron. Under no circumstances are you to reheat the left overs.

Emergency Shews:

Bubble gum that turns into one time use sandals. Once the flavor runs out, you have 30 seconds before the gum expands into shoes. The sizes seem to have been printed on the gum but they’ve long since faded or rubbed off. Durian flavor.

Dead Cat Bounce:

A black bottle with a cat eye painted on it. If you drink the contents and die due to falling from a great height, you will be revived immediately and launched with equivalent force in a random direction.

Cape of (Refugee) Flight: you gain the power to fly for your life.

Screaming Cloak of Invisibility: you’re invisible, but the cloak constantly screams, “HE’S OVER HERE!!!” and tries to give away your location.

The Tax Axe: raises both your taxes and your target’s with every swing.

Bag of Communal Holding

Content is shared with all other bags of communal holding in existence. Sometimes retrieving objects involves awkward hand contact if someone else is using their bag at the same time.

Bag for holding:

It’s a bag of holding but instead of occupying a bag slot it must be kept in the main hand.

I have a spreadsheet full of these somewhere. The 2 that my players got that I remember them using are:

Emperors armor: +3 full plate armor with no strength or armor proficiency requirement. When you look at yourself or in a mirror you see yourself in full plate armor, but to everyone else you are naked and if anyone tells you that you are naked the armor and all your possessions cease to exist. My player made it the shopping district in the heart of the city before someone told him.

Staff of disintegrating: when activated, it disintegrates. The player that got this one saved it for a boss fight. He found it hilarious though.

Ring of invisibility. Makes you invisible, but makes everyone else invisible to you as well.

Circlet of human perfection, but the creator had a really niche fetish.