Hei there 
My third weapon slot is locked when selling items to the vendors.
I don’t know if this information will help as well, but I’m in the endgame, not all vendors haven been upgraded to the maximum yet.
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Thank you four your time and effort you are putting into bugfixing, it is a great game and I am enjoying it very much.
Main hand slot #3 shows locked symbol after purchase. the weapon in the slot can be unslotted and reslotted to fix the glitch, however once reopening inventory must perform this again. This also is present when viewing a merchant’s inventory, the item in main hand #3 is unavailable.
Same bug like photo. Both my 3rd slots actually open, even there are weapons, but at Vendors and at Chests, its not showing 3rd slots . Looks LOCKED.
Also BODY Armor should be top right, its going bottom left (changing slots with pants slot) you can see that at photo too.
Fix please
Even with Ichor’s character completely unlocked, when interacting with a chest, even with the weapon/shield slot unlocked, it is still blocked when interacting with a chest.
youtu.be/eK77rMNtw3I
Same issue here. Persists through realms. Haven’t gotten around progressing another char that far to check if it’s universal.
So the problem is that when in a chest or even shop interface you can not see the last main hand or off hand slot even though I have it unlocked and things are on it..
Yea so i have that too … i see them when i look at my inventory. But not in the shop window or when inspecting chests
When opening a chest or in a vendors sell menu the third weapon slots still show a dot (saying its still not unlocked) even though in character menu i have a weapon equipped in this slot.
Current work around is unequipping it to put it in menu before storing it in a chest. A minor QOL but just wanna make sure your aware of it.
I also had this bug. I believe it was because I added my third weapon slot in another world besides my main world, with a secondary character. What initiated this permanent error, however, was my 3rd slot weapon equipped, a rusty sword, vanishing from my hand soon after during a bounty. i quit game, reloaded and unequipped weapon from third slot. however, i quickly found whatever weapon i did equip in the 3rd slot was invisible in the menu.
i quit again after unequipping everything, reloaded and then everything was visible in the personal menu, but vendors and chests/storage do not acknowledge that there is a third weapon slot. i can still rune and enchant the third weapon, but cannot store or sell it, unless previously unequipped.
one potential solution i will attempt to get the 3rd weapon spot back visible to vendors and storage, is to be totally unequipped and empty of any items the next time i level up w plague ichor. i will also expand my 3rd secondary item slot to see if this helps.
have the same issue, hope they fix it soon, it was present before patch 2 
the first time i saw that i jumped on my seat, i had a legendary GS on this slot lmao …
there is a similar post somewhere about item slots being lock in vendor menus.
found it: Third weapon slot does not show in menus
As in subject - both mainhand and offhand slots look as if they weren’t enabled.
There are two other threads, regarding this problem.
I will link your post here: Item not displayed
here is another post regarding this issue: [bug] when browsing chests, main/off-hand enabled 3rd slot is locked
Here you can see the video. I have 3 characters, two of the three have everything unlocked but not the third. This is probably happening when I switch from one character to another
Oh, that’s a problem, it’s been there since I started playing a month ago, I didn’t even bother to mention the bug, after all, I basically don’t use the feature. Let them fix the important ones first.
