Guild Wars 2

How do you know what upgrade components to purchase from the trading post ?

The reason I ask is sometimes when I buy an upgrade component for the level I am at it won’t work with the weapon or armor I have.

Runes can only be placed on armor, Sigils only on weapons, Jewels only on accessories.
Gemstones (the ones that are weaker than the Jewels since they aren’t upgraded by a jewelcrafter) can go on any piece of equipment, as can Marks, Talismans, etc etc.

Also when slotting its the equipments level not yours that is used (taken from the level requirement)

You cannot put the highest quality gem in a level 1 (or no level requirement) item.

So if a gem says req level 20 the item you want to place it on needs to have a level requirement of 20 or higher

Thanks for the replies.