"I want to begin a legitimate business online for reselling items on eBay and Amazon. Obviously I want everything to be legitimate and want to pay or report all necessary tax information when the time comes. How do I go about doing this?

Keep correct records of cost of items, cost of business expense. Then record income and figure profit.

You will pay taxes on the profit.

You may want to talk to an book keeper or accountant on some simple steps

If you have no employees, you can just be yourself, a sole proprietor.
Get a sales tax ID from your state tax or revenue dept.
Collect sales tax on sales shipped to your state only, for now (laws keep changing). Your state will provide instructions for when to pay.
Track all INCOME and all EXPENSES.
Print out forms from IRS.gov for next January, a long form 1040, Schedule C, and Schedule SE. They are easy to fill out and pretty self-explanatory.

Nobody wants taxes on used items around the house, unless it's a full time operation like a second hand or antique shop. Many sellers start that way for experience. There is a lot to learn. All the ins and outs of shipping are vital. There are MANY ways to make mistakes, be scammed, or to simply cost yourself more than you make.

Helpful addition on consumer end of things (I've recently been claiming out-of state purchases, during my annual, February trip to H&R Block.)
We pay the consumer taxes for ebay/amazon items, at the end of the year.
This is done as a verbal & well educated, guess(no receipts).

I can't make any sense out of the last response. And there is no room on a tax return for guessing, with no receipts.