I have a paladin who unknowingly bought some poor quality hand axes from a festival.  well in the middle of combat the hand axes broke.  Being a paladin he (the player) said that his character would want to confront the salesman.

i have it to where the towns' festival coordinator gave them the location of the salesman, who most likely sold the paladin the goods. oh look, it happens to be on their way to the next town.

Well the salesman is a definitely a "snake oil salesman" who sells poor quality equipment for large profit.  

Here is where you come in.  i need help coming up with the "poor quality items".  for instance: 

plate armor that is super heavy.  it gives the paladin plate armor +1 however its so heavy, the wearer couldn't move as fast so -5ft of movement and -4 on dex saves/checks/attack roles that rely on dex

a longsword that is made of soft metal, when the character is exposed to fire damage the sword takes damage as well.  50 total points of fire damage destroys the sword.  allowing it to be considered a "light" weapon

a light crossbow that misfires on a natural one and becomes jammed

What are your ideas?

Cherry pick some trinkets from the trinket table. There should be some that feel really magical when you describe them, but are utterly useless in entirety.

I love it, because a friend of mine is running a campaign that I wrote for him, and I'm playing in it (it's ok, I forget everything I write, and usually use DMPC's people like). A player got a trinket that was a warm shard of obsidian, and he is hoarding it, trying to not let anyone else have it, because he thinks it's for fire magic or something along those lines. I can tell, because he min maxes, and doesn't know about trinkets.

Most poor quality stuff is rather boringly poor quality -- in D&D terms, breaking easily, penalties to ability checks with tools, penalties to attack and/or damage with weapons, penalties to AC or mobility with armor.

Trinkets are definitely a good idea! You could also have some elixirs (aka, potions) that claims to have great effects but...are more lackluster in practice.

Elixir of Attractiveness. Drinking this elixir every day for a tenday will guarantee a brighter, more attractive you! Actual effects may vary, including turning the user bright pink with a soft glow which would certainly  *attract* quite a bit of attention. (Effects wear off in time)

Elixir of Good Health. An elixir a day keeps the cleric away! This orange drink has a pungent citrus tang and is most likely just a shot of vitamin C but most certainly gives one a sense of nutritional superiority.

Hat of Poor Omens. This woeful item will bring misfortune to any who wears it. A great gift for your rivals! Actually just an itchy wool hat with lice in it.

@pantagruel666 how would you make them more interesting.  By definition, these items that this particular NPC is selling is not magical, so i am interested in what else you have in mind. 

The Paladin should be ashamed he did not know quality weapons. 

My level 2's got to their first big town and immediately started in on wanting vendors to spend their stack of 10 to 15 gold.  

So the Ranger found a guy selling really shiny scimitars. That were very lightweight. Yep wood painted with silver.  Ranger managed to haggle him down to his whole supply of 15 gp.

Another guy bought a magic necklace that would prevent a monster from killing him. . And immediately put it on. So as soon as he made an aggressive action the choker came to life and started cutting his neck off. 

Bows and arrows that seem fine, but as soon as there's a little rain or moisture or the player gets dunked in water, they all warp and become unusable.