What do you guys usually buy in the auction house?
Haven't bought a single thing yet, but sold some..

When do I start making serious dough? like, at what lvl? Currently lvl 16

You can buy some gear to make the leveling process easier, but it's better to save up as much as possible for lvl 80. (including karma as well btw)
Just make sure you don't outlevel your gear too much for now

Alternatively you can save up some more gold, then craft for a few levels. I went from 75-80 in 30 minutes by spending some of my gold on crafting mats.

Alright, cheers.. I haven't done any crafting yet, might wanna look into it.

Is it an efficient way of leveling from 15- ?

Also, whats sigils?

I buy stuff that will make me more money

Crafting is VERY efficient at levelling, but it is extremely costly, especially when you start running into multiple disciplines (cooking, artificing and jewelcrafting are the cheapest)

Sigils are like Runes but for weapons.

Thanks