To vet a supplier properly you do three things in order: verify the business actually exists and does what it claims, test them with a sample and a deliberately detailed conversation, then structure the first order so a failure costs you little. Most supplier...
The way to find a product to sell online is to run a filter, not to wait for an idea. You generate a wide list from places where real demand is already visible, cut it against fixed written criteria, then test the two or three survivors with a small amount of money...