ljbike

Anything you buy from a wholesaler is paid for at time of purchase; either with a credit card or on a COD basis unless you can qualify for credit which then gives you 30 days before you have to pay.
Before you can do business you need to register with your state's tax office and get a sellers permit; register with the federal government to get an employer ID number; get permission from your local town to set up the business; get a business account with your local bank. Then you have to prove to the wholesalers that you really are in business and not just some guy trying to buy parts cheap.
Other than that, there's not much to it except it also helps if you know a little something about merchandising, advertising, retailing and how to deal with customers who all think you are a chiseler. a cheat. a liar and a thief and want you to sell them parts as cheaply as the internet or the discounters -even though you may have to pay .50 cents less than they are selling it for. It also helps if you have a working knowledge of the many different kinds of bicycles and the many different brand names and the quality of each; because when the hordes of bikers come streaming into your store they will be full of questions and expect you to know the answers. And they will expect you to know how to fix their bike yesterday so they can ride it tomorrow on a 150 mile charity ride that they've been soliciting funds for and have been too busy to bring the bike in before now. And you'll need to know how much to charge for these services.
And then you get to go to Las Vegas in the Fall to the Interbike Show and see all the new bikes and gadgets and see the show girls and gamble and hit the jackpot and get rich and retire and hire someone else to run the shop for you.
Ten years ago there were over 9,000 bike shops in the USA. there are now barely 6,000. You can start doing all this with as little as $10,000; or set up on a corner like Joe's friend. Good Luck.