There are no certifications that are required for computer repair. However, if your business gets heavily into networking and starts cultivating larger business clients, then it may be valuable to have an employee that has some networking certifications.
IMO start with Newegg as a supplier. Some of the larger distributors have pricing tiers based on volume, and the lower tiers end up costing more than the cut-rate e-tailers.
As mentioned, keep a slim inventory (especially at first). Wouldn't hurt to have an optical drive and medium sized hard drives in stock (both EIDE and SATA), plus ALWAYS have your service technicians carry some popular RAM sticks with them on house calls. Those make for quick and easy sales. A handful of various PSUs are also important to have on hand, but make sure you don't get ones that are too crappy. Everything else can be ordered as needed. If system sales pick up, have "stock" configurations so that you can have a barebones ready and then just slap in the RAM/CPU/HDD needed.
You don't need to offer a huge selection of brands because most people you service won't be enthusiasts, so as long as you are comfortable using a part in your own system then go ahead and use it in a customer's system.
There is little markup in hardware. If there isn't much competition in your area you may be able to get away with 10-20% markup, otherwise you may be lucky to get 5%. Don't try to rip off anyone (such as doubling the price of the store down the street) because eventually some of them will find out, and any that do will be a lost customer plus bad word of mouth.
Service is your profit. Do not give it away, because once you do then that customer will come to expect it. You are charging for your expertise, knowledge and experience. I'm not a fan of anything "flat rate" so IMO tier your service in time increments (hourly or half hourly).
Do not BS the customer. If I really don't know something I will tell that to a customer. I have NEVER had one customer belittle me for it, and have had many praise me for my honesty. Long term customers always knew to come to me for real answers because they didn't have confidence in the answers my business partner gave (BITD when I did that for a living) since he would BS when his knowledge ran out.
Techs who can "figure out" stuff are valuable because not only do computer technology change so fast, there are so many possible things that can go wrong among the dozens of brands of hardware and billions of lines of code in a computer interacting with each other that many things won't go "by the book."
I'm not a fan of fanboyism (har har). While I may give strong recommendations, I can usually back it up with more than just "because the other stuff is crap." My business partner (and a lot of other techs I've met) were really opinionated and that seems to draw the less knowledgeable clients because most of them thought that someone with a strong opinion must really know their stuff. However, I've noticed that was a turnoff to other clients.