Aval Atheron is a dunmer merchant who has a market stall in Windhelm. Merchandise displayed in the suggests that Aval sells meat, yet vanilla game doesn't reflect it. The issue was somewhat resolved in Unofficial Patch, making him a solely meat merchant, but it ignores in-game hints about Aval.
He is a pawnbrocker, as reflected in his dialogue with Tova Shatter-Shield: 

Tova Shatter-Shield: "I understand you purchase used goods, as well as sell them?"
Aval Atheron: "Yes, that's right. Whether or not I'm interested depends on what you've got, of course."
Tova Shatter-Shield: "I have some things that belonged to my daughter, Friga. She... she won't be needing them any more."
Aval Atheron: "Oh, I see. Yes, yes of course I'll take a look. Just bring them by whenever it's convenient for you."

And he sold some pitchforks sometime to Torsten Cruel-Sea: 

Torsten Cruel-Sea: "Did you have any luck tracking own some more pitchforks?"
Aval Atheron: "Revyn put the word out to his suppliers but with the war on, tools are scarce. Both sides have been snapping them up."
Torsten Cruel-Sea: "Tell him I'll pay double the normal price. We've been getting by with one broken pitchfork, but it's falling apart."
Aval Atheron: "I'll be sure to stress your urgency. It seems like every farmer in Skyrim is short on supplies these days."

Despite that, his barks usually mention him selling food:

"Assorted fruits and vegetables, all delicious and affordable!"
"Come and browse my fine selection! Everything's fresh and delicious!"

But if you ask him where does he get his goods, he answers:

"Wherever I can find them. If you're the type who worries over where something came from, you may want to move on."

...which really doesn't scream "Hi I sell goat legs" to me. 
So, my interpretation is this - Aval Atheron is a shady dunmer merchant who uses a meat stall as a front for his slightly illegal pawnbroker business, and as such completely removing everything except meat from his inventory is an incorrect solution.

This mod changes Aval's merchandise to something between vanilla and unofficial patches versions. Now he will sell and buy jewelry, enchanted and not, some clutter items, clothes and lockpicks - stuff you can imagine people of Windhelm actually bringing him. He will also keep selling meat from his stand, as introduced by unofficial patch.

INSTALLATION

There are two mod files for you to choose from.
Regular one changes Atheron's merchandise, and Stall Remake version modifies wares on display in his stall to reflect the changes. 
Stall Remake can be incompatible with City Overhauls, but I hadn't encountered any major issues.

• Does this require unofficial patch?
  

No. While it reverts some changes made by it, it doesn't need it.

• Do I need this if I don't use unofficial patch?
  

Mod's merchandise is different from vanilla. In vanilla game Aval sells weapons and armor, which, while not completely nonsensical, seems meaningless and highly illegal to me.

• Is it safe to install mid-game?
  

Should be, but it will take some time for Aval's inventory to update.

• Is it safe to uninstall mid-game?
  

Don't uninstall mods mid-game. But probably yes, it is safe.

• Is this esl-flagged? 
  

Yes.