Seeing that you are based in Spain although you will be selling in the US (via dropshipping I presume) does not qualify you to get a US resellers ID or Tax ID.
You will need a resellers ID if...
You live in the USA or Canada. Most states have a sales tax, but there are a couple that don't. If you reside in one of these states just let the wholesaler know and they may have a different form or requirement for you to complete.
You sell online to make money (i.e. You sell more seriously than just getting rid of a few old things from around the house every now and then). You need a seller's permit so you can charge and collect taxes, which you'll pay to your state government later.
You want to purchase goods wholesale from a US supplier (and you are also based in the US), you need a seller's permit to buy wholesale sales tax free.
You may want to get a VAT ID instead as this is the equivalent in Spain.
Hope this helps!