id stick to 1 items, like b rings, gwd items, etc.
bulk items are much harder to sell off and are only worth it if your buying for a cheap price imo.
I'd disagree hugely. It depends whether you're looking to make a quick buck by flipping an item (in which case you'd settle for an expensive item like a GS, DFS, etc.), or whether you actually want to merch. One major misconception with merching is that the more expensive an item is, the more money you can make with it - this couldn't be further from the truth, in my opinion.
For example:
I find someone wanting to offload their DFS to buy something else, so I buy a DFS cheap for 26m, resell to current most recent buyer on zyb (at time of posting) for 29m. I've made 3m profit.
OR..
I find someone selling an item in bulk, such as runes, arrows, ores, etc. For an example I'll take soft clay, because it's the item that I've made possibly the most money from. I've negotiated a bulk seller down to 100gp per soft clay previously, meaning for the 26m spent on a DFS in the previous example, I buy 260k soft clay. I then spend a little more time finding a bulk buyer, and resell that clay to them for 150ea, making 39m (of which 13m is pure profit).
That's a real example, with the soft clay - I've bought and sold millions of bulk items, and it has made me lots of money. But I steer totally clear of high value items because they are massively less profitable.
If I had to give one piece of advice, it's to get to know your buyers and sellers. Taking the time to get to know my buyers and sellers in the past has led me to find even more buyers and sellers. I've had regular sellers sell me their bulk items and then tell me the names of people who had also wanted to buy it from them, so I could turn round and resell it to them for a profit. You're unlikely to find someone who will sell you a godsword and then say to you "oh yeah, this guy had offered me Xgp more so you could resell it to him".