OK I encountered a very annoying problem with all the fences in the game.

When I buy items from them the transition works fine, their gold increases appropriately and is taken from me and I get the item. When I try to sell though, their gold does not decrease, I don't get any gold but the item is still sold. Keep in mind that none of the items I tried to sell were stolen. I only noticed this with fences as no other merchant seems to have this problem. Also I don't know if it's a game bug or something to do with a mod. So here is a list of all the mods I use, these are all the active plugins:

00 Skyrim.esm
01 DeadlyDragons.esm
02 MatchingSetFixes.esp
03 Bag of Holding.esp
04 Headbomb's Better Sorting - Miscellaneous.esp
05 Headbomb's Better Sorting - Ammo.esp
06 Headbomb's Better Sorting - Soul Gems (PseudoVanilla).esp
07 Headbomb's Better Sorting - Food (Reduced Weight).esp
08 Headbomb's Better Sorting - Books.esp
09 Headbomb's Better Sorting - Ingredients.esp
0A Headbomb's Better Sorting - Potions (Weightless).esp
0B Headbomb's Better Sorting - Spells.esp
0C KillThemGenerals.esp
0D More Craftables.esp
0E Dragonbone Weapons.esp
0F Dragonbone Weapons - Deadly Dragons Compatibility.esp
10 Geralt_RavenArmor_v02b.esp
11 TrissArmorRetextured.esp
12 Complete Crafting Overhaul - BS version.esp
13 DeadlyDragonshardcoe.esp
14 DeadlyDragonsCraft.esp
15 Proudspire Manor - Dragonborn Edition.esp
16 Weapons and Armor fixes.esp
17 Weapons and Armor fixes - Hacks.esp
18 Essential Meeko.esp
19 Essential Vigilance.esp
1A HoodsAndCirclets.esp
1B Follower Healing Potion and Arrows.esp
1C LegendarySmithingUpgrades.esp
1D LFox Alchemists Have Ingredients Medium.esp
1E moveitLWT.esp
1F No_dog_meat.esp
20 No NPC Greetings.esp
21 no_quest_items.esp
22 RealisticWaterTextures.esp
23 RichMerchants.esp
24 Smithing Perks Overhaul - Balanced.esp
25 AdvancedWerewolf.esp
26 True Vampirism - Original.esp
27 Enchanting Freedom.esp
28 IPutASpellOnYou.esp
29 Balanced_Alteraton.esp
2A Balanced_Conjuration.esp
2B Balanced_destruction.esp
2C Balanced_Illusion.esp
2D Balanced_Magic_Adept.esp
2E Balanced_Restoration.esp
2F LovelyHairstyles.esp
30 Better Followers - SP.esp
31 Height Adjusted Skyrim.esp

EDIT: It seems to be happening to other merchants as well. I was buying ingredients from Angeline's Aromatics when she reached around 47k gold. I then exited the store and then remembered that I wanted to sell stuff so I went back in, and again no gold transition.

I once noticed this to happen too, but afterwards on another merchant it didn't happen, so I just forgot about it.
Since I'm not into the Thieves Guild questline yet, I don't know what merchants are fences, so when that happened I probably was selling items to a merchant that is also a fence.
No mods or tweaks here, just vanilla Skyrim.