You can't do the quest as a single player. This is a bad game
design.
Yes it would be bad game design if this was a single player game! But it's multiplayer and interaction with other players is must... So this is not a game's flaw.
As for the original question.
Crafting quests are given to you is to craft an item (the game selects one of your already learned recipes, not your secondary profession!)
Gathering quests are given to you is collect raw material (the game checks your secondary profession in order to chose the material!)
At first case:
You have learned a recipe means that you are able to craft it! But many recipes require a lot of resources that we are not able to gather them all by ourselves. (such as wood, hides, stones etc) what you do? 4 ways to obtain them.
1) Join/create guild and exchange materials inside the guild
2) Buy them from the market (auction is instant when you are buying doesn't take time as previously someone said)
3) Use several accounts (multi-account doesn't conflict with game rules)
4) Attack other players who are gathering resources to dangerous/no-man's land provinces hopping they are gathering the resources you need and then find someone to craft them for you (again second account or guild may required!)
