If you’re planning to resell on demand, I’d look for a provider that offers reasonably stable configurations and pricing, but also has enough inventory to handle orders as they come in.
One thing worth asking potential vendors is how they handle discontinued hardware or pricing changes. Some may be able to offer comparable replacement configurations at similar price points rather than forcing you to continually update your own offers.
For dedicated servers and GPUs, I’d also ask about provisioning times, available locations, bandwidth options, IP allocations and whether they have reseller or volume pricing. VPS is a little easier to keep consistent, but dedicated hardware and GPUs can change quickly depending on inventory.
Are you looking primarily for US locations, or do you need Europe and Asia as well?