About the tradersand merchants

I know the traders only trade items that they are also sold. So, the rune traders can only sell the runes that the players sold to them.

What I'd like to know is whether items sold to merchants are also available to the traders. Do I "harm" the trader's stock if I don't salvage insignia and sell them to the merchant?

Probably, because what your selling is an armor piece, not a rune, insignia, etc. But if you salvage it and then sell it, you're "harming" yourself by not being able to get a good amount of gold.

However for some items, like mesmer runes and granite, the merchant offers the same price as the trader (at one stage the merchant offered more). Since the you can sell them to the merchant faster for the same price, selling to the merchant makes sense. But until you become familiar with the various prices, its best to check the trader before you salvage the insignia/rune. If the price the trader sells it at is 100 gold then merchant it (if you feel lucky you can try salvaging, since successful removal of a rune/insignia won't reduce the price of the remaining piece), but if its over 100 gold then salvage it out to sell to the trader.

Now while I don't think that any runes/insignia's sold to the merchant go into the traders stock this doesn't really matter as for a trader item to have its price drop to 100 gold requires an excess of that item, meaning that selling it to the trader shouldn't have any averse effects on the overall economy. Not that one person can have much influence on the economy anyway.