A domain name is www. your website .com
You only charge sales tax if you sell to someone from your state.
A tax ID helps with finding suppliers, you can go to Link hidden: Login to view for your EIN tax ID.
Here is a couple places to start.
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Also try buying from ebay. I buy an item on ebay for $39.99 and my supplier, that I buy from all the time, charges $69.99 for the same item. I sell the item ON EBAY for $139.99. This item only comes up on ebay every now and then so I have to watch for it. I don't know how they can sell it for so cheap but I don't care.
Check the stores in your home town. I was in Meijer and found beach towels for $2.50 a piece so I purchased all 100 of them and sold them for $9.99 a piece. The guy at Meijer even said that they pay more for them than I did. They were just clearing room for this years towels and had to get rid of them.
I just started selling in January of this year, so I don't know too much but I do remember what it was like starting out. (Watch for scams) if anything seems too good to be true it probably is. The sites I have listed, you can trust.