Most first bulk orders go wrong for the same reason: the buyer evaluated price and product, but not the supplier. Price is easy to compare. Whether someone answers an email in March, when your container is stuck at port, is harder to test — but you can test it.
One sample tells you what the product can be. Two samples, ordered weeks apart, tell you what it usually is. Consistency is the whole game in resale: if the second batch looks different from the first, your customers will notice before you do.
Ask a question that requires real work to answer — a spec question, a customisation request, a shipping quote to an awkward destination. How someone responds when there is no order on the table is how they will respond when there is money on the table.
"About two weeks" is not a lead time. "Dispatch by the 14th" is. Vague timelines are usually a sign that the supplier has not checked their own production schedule, and you will be the one explaining the delay to your customers.
Every supplier ships a bad unit eventually. What matters is the policy: who pays return freight, whether replacement or credit, how long you have to report it. Ask before you need to know.
A supplier serving established resellers has usually been through this process before and has systems for it. A supplier who only sells direct to consumers may not have a wholesale lane at all — no tiered pricing, no quote process, no account manager.