How?
They are the opportunistic middlemen between consumers and sellers making profits off someone else’s product/work/service for their benefit.
If you pay resellers, you are paying them $$$ or whatever amount for their benefit that you could have done for…$0!
Some resellers are legit, selling authentic stuff. Some resellers are shady and not selling legit stuff.
Buying direct from the source eliminates this risk.
If Nike limits their products, it is their prerogative, and that is not good for consumers but who takes advantage of that? Resellers. Nike is partly to blame, besides resellers, when they limit the supply of their items that have a larger demand.
The resellers who are buying up the latest releases are annoying and problematic for buyers. If you go in store and wonder why they don’t have a full size run, well, blame the resellers and bulk buyers. Don’t blame the consumer for that.
The resellers who happen to sell vintage, hard to get, hyper strike, tier zero, one/one type of items which are not necessarily available to the public can be valuable to some buyers who are in the market for that.