As a general rule, drop shippers won't require a tax id, but genuine wholesalers will. Of course when you buy wholesale, you buy in volume and pay less per item because of it, so wholesalers do require an account.
In regards to being a SP, that's straight forward. Just keep all your tax records nice and clean, and that's it. Always a good idea to speak with an accountant in setting up any business, so I would suggest that be your next step and they will be able to inform you of your taxation responsibilities.
Good luck with the new venture!