Anything, where the demand will outpace the supply is a good bet.

The only tricks are identifying the items which will be in demand.
And selling when the most people want them.



I personally think, the majority of new toons will have a main with a fair bit of gold.
And prefer to buy 'x', rather then slow their leveling pace and farm it.
Particularly, with 800 other new toons in the same zone also farming.

I'm not infallible, and could be wrong on this.
I've got 30 stacks each of Peacebloom, Silverleaf, Briarthorn, Copper Ore, Tin Ore, Linen Cloth, Wool Cloth, and am working on Strange Dust.
A tiny bit over 1000g on mats, which I anticipate selling a large mark up.
But if they don't sell in the 25-100g a stack range, it's not like 1k is that big of a deal; and they'll sell for something.