Why vetting comes before price
A quote is the easiest thing to compare and the least useful thing to compare first. Unit cost tells you nothing about whether a factory makes your product. It says nothing about the certifications your category requires, or whether they will still answer once your deposit clears.
Vet first, negotiate second. A supplier who fails the screen is not a cheaper option. Treating them as one only moves the cost somewhere you cannot see it. A missed production window, a failed inspection, a listing suspended for documentation you were never sent.
Where to find manufacturers
Most first-time sellers find suppliers through one of three routes. They differ in reach, and in how much verification they do for you.