Check the marketboards for things that are relatively easy for you to craft or gather. Craft/gather them. Sell a bunch. Look for niche markets - the stuff everyone's playing in is, inherently, going to be a really unstable market. No harm in jumping in yourself if demand is high, but I wouldn't bank on making it your 'thing'.

I also really recommend you diversify. Devoting all 40 retainer selling slots to one type of item is a great way to get burned when some complete goon comes along and undercuts you by 1,500% and crashes the market. If you play in a bunch of different markets, you'll be a lot safer when (NOT if, WHEN) they get crashed.

Beyond that, specific advice is hard. Every server's economy is different, so any advice someone gives you is going to be biased towards the server they know. What's worth a ton on one server is cheap on another. You've just gotta find your market yourself.