You will learn very fast that you don’t have much leverage with suppliers and manufactures early on. You’re probably too small for them to care or waver outside of their normal operating procedures. The best advice I have is to build relationships and tread lightly. This whole game is about relationships, from manufacturers willing to do favors to buyers knowing a friend of a friend and making the introduction. Always put yourself in a place to foster relationships, especially because this is a small and old-world industry. If you have the option of working with many different factories early on or a single factory (assuming the single factory can produce at a cost and quality close to the many different ones), go with the single factory. You will learn more and you’ll start a relationship you can expand on in the future. Business is also business, and the more you can bring under your own name (or by referring other clients) the better you will be treated.
But often times, you won’t have any leverage and negotiations turn more into monologues. You’re going to have relationships that start off promising and that seem like they will last. But then they will dry up. People will stop returning phone calls and texts; it will feel like they disappeared. It's terrible, but it happens. Persistence is key in this world, but so is knowing when it’s over and time to move on. This isn't something you will ever perfect, but you’ll learn to better feel it out as it happens. Having backup plans and redundancies is crucial here because people will drop off the face of the earth, take an unexpected vacation, or just be unreachable, and at the end of the day it’s still your problem. This is just the reality, and be prepared (as much as possible) to work with it.
This is, at the end of the day, a pure execution game. Everything is stacked against you early on, but figuring out (often the hard way) how to maneuver around it all is what will set you apart.