On a semi related note I think it's important to understand the types of goods you are aiming to supply.
Type 1 : Exclusive brand access, unlimited stock - insanely valuable (rare)
Type 2 : Only source of a listing but not necessarily exclusive - insanely valuable (short term)
Type 3 : Distributor stock - hot items, clearance items, replenishables - offers consistent okay returns, time consuming, competitive
Type 4 : Rare hard to find items - great profit to time ratio, limited stock, high risk, exclusive access?
Type 5 : Flippers - low barrier to entry, highly volatile, short term gain, time consuming to track trends.
I personally view items I sell in these buckets. I currently use type 2/3/4 and very rarely 5. I spend most of my time working towards finding type 1/2 connections. I have a couple of type 2 sources setup and I can't stress how much easier life is. You place a huge order, label it, pack it, send it and forget. No deal hunting, no price wars, no store hunting. I'm not going to quit my day job anytime soon but with enough of those connections, or the right one, you have yourself a long term repeatable lucrative future.