I can't be objective since I haven't done this in more than an year, but generally you want to spend some time lurking the item board and observing the prices, you would also want to know the frequent users and how they tend to purchase.
I'm not sure how the restock works now, but it's a good idea to check out torishop stocks, and consider in what part of the restock cycle you are to adjust your price accordingly.
By knowing the frequent users more importantly you will know when someone infrequent posts and you can earn a decent amount of off him.
Generally it's a good idea to avoid gradients,timers and non-force/lax items unless you are opening a junk market/collecting sets in which case it is great.
Avoid buying stuff, from frequent marketeers, because probably the price they give does not suit you to resell, unless you have a PM from someone with a better price in which case it's instant-cash.
This is it in a nutshell, I've possibly missed out a lot of things, but mainly you should know the other visitors.