Merchanting advice

I'm trying to make some money on the side while I skill (mainly fishing) and I want to try merchanting. I've merchanted small stuff like cowhides, iron bars, etc.. it's not that much profit. Maybe 250k-500k a day if I'm lucky, but still not bad for no work.

I'm investing in some range ammies atm, bought three of them (used up all my cash on my n00by account)

But honestly, it's just a guess. I'll probably wind up losing money on it. Any way to tell if an item is going to suddenly rise? NOT merch clans >_<

Any advice is appreciated.. more looking for how to tell/guess what to merch, not a specific item.