In the patch notes for World of Warcraft: Shadowlands patch 9.2, some changes crept up a while back. When trading gear items in Patch 9.2, the developers once again clarified how the rules are defined when trading tier set items and provided an update to the legendary item slot.

The official post on the US site reads:

Legendary gear no longer counts toward unlocking the highest item tier for that item slot for trading.
Class set items cannot be traded until a soulbound set item of the same or higher item level has been purchased in the respective gear slot.
The peculiarities of the personal loot system.
The personal loot system introduces some confusion. Players can only trade an acquired item if they have an item of the same or higher item level in the same gear slot. For example, if you receive item level 252 gloves and only wear 232 gloves yourself, you won't be able to trade them.

More on Warcraft Patch 9.2
WoW WotLK Classic: Ulduar - Brann Bronzebeard
WoW WotLK Classic: Pictures from the 2008 launch
WoW WotLK Classic: On the occasion of the upcoming announcement of the Classic remake, we take a look back with a picture gallery!
World of Warcraft: All class buffs and nerfs for the next ID change on April 6, 2022
Tomorrow, April 6, 2022, a slew of class buffs and nerfs will go live in World of Warcraft. Here are the patch notes!
So it's always important to equip the highest item level item immediately, even if you don't wear it later in combat. Items in inventory only count as equipment when they are actually equipped. Players who receive new rings or trinkets will immediately drag them into both slots for the system to recognize, and subsequent rings and trinkets will be honored for potential trades.

Why make this change?

Changes to the settings item have been clear for a few weeks, but it's important to reiterate. Only those who already have matching set items in the same slot can pass their level set items to other players next time.

These two changes are primarily aimed at hardcore guilds who want to focus their gear on a single character on the main team via split runs so they have the highest item levels. Layer set changes make this more difficult. Buy WOW TBC Classic Gold now.

A well-known trick is to create a legendary item with a high item level once so you can trade the next item from the raid or dungeon in the corresponding slot. Professional guilds invest heavily, sometimes creating a legend gap only once. The second change to Legendary items is to prevent players from wearing 291 Legendary Blanks in every possible location, in order to be able to trade as many items as possible. If you want to buy WoW gold or WoW TBC Classic Gold, you can find it in MMOSO, which is absolutely safe and reliable.