In regards to Tax ID requirements, all wholesalers will require a Tax ID (Reseller's Cert in USA terms) to sell to USA based businesses because they are withholding tax, in other words selling at wholesale price. Now, if you are a business based in another country to that you are wanting to buy wholesale from, that Tax ID is not relevant in most cases because you are importing the goods to sell in markets outside of where the supply comes from.
Where it may be relevant is, if that business then wants to sell that product back into the market where the supplier is actually based. Depending on what the product is and what country we are talking about (because every country has it's own confusing tax code), you may be required to submit a Tax ID under that circumstance.
In some cases, International buyers may still be asked for a Tax ID, despite being an International buyer. The easiest thing to do then is to just provide them with the equivalent Tax ID from your own local Tax Authority, this will usually be quite acceptable, but in many cases, once a wholesale supplier understands that you are an International buyer selling into International markets, then the Tax ID is not relevant for them.
In regards to Drop-Shippers, you may find some that request a Tax ID, but it's really not relevant in most cases because Drop-Sipped goods usually do not have tax withheld to start with, as does wholesale prices.
Now you're quite correct, buying wholesale does give you a better price because you are not paying the tax component and you generate a much better price because you are buying in volume, as opposed to Drop-Shipping, where you are generating sales on a per item basis.
On top of that, you have the supplier warehousing and then picking, packing and shipping the item for you also, which of course carries a fee. This is a part of Drop-Shipping that a lot of people don't really consider and completely overlook when they wonder why Drop-Shipping prices are so much more expensive than wholesale.
So that's where all the profitability usually evaporates to when comparing the skinny margins on many Drop-Shipped items in comparison to wholesale products.
I'm looking forward to getting clarification as to why the member is getting rejected, I have a feeling that there is some misunderstanding going on that we should be able to clear up, but I just need more information to be able to pinpoint it for them :)