Please note that some of the services I recommend are affiliate linked, meaning I may earn a commission or bonus if you purchase them.
You may be wondering why I would put Ebates before eBay or Amazon and that's a great question! The reason is that, no matter what kind of business you want to run (or if you don't want one at all), you should be using Ebates! Seriously, if you do any online shopping - ever - it's well worth having an account. It's especially good for retail arbitrage and you get cash back on literally every purchase. You'll also get $10 just for signing up above! Win-win-win-win-win here!
I started by selling on eBay, but Amazon has helped me grow my business exponentially. It's possible to have an online retail business exclusively on eBay, but it's not near as profitable as focusing on Amazon, or both. I've found that different things sell better on each of the platforms and that there are benefits to each. Regardless, you really should have an Amazon seller account. It's $39.99 per month, but you can usually get your first month for free to try it out.
You may already have a PayPal account, but you will want to upgrade to the business version. It's free to change (or sign up), but it's important to do so because the business version saves your statements so that you can access them months later, whereas the regular personal version does not. It's important that you can get this financial information for tax purposes.
You don't have to get a UPS account, but it's helpful, especially when you start selling through Amazon FBA. Amazon primarily contracts with UPS and they give great discounts on shipping larger items via their system. You'll also need an account in order to schedule pick-ups, which will be really helpful when you're shipping 20+ boxes to Amazon warehouses at a time...don't worry, you'll get there!
Even if you don't plan on selling on eBay, you should have an account. The reason is that many popular stores and brands now have eBay store where they sell items at larger discounts. You can have some great luck with retail arbitrage, order in large quantities, and then turn right around and sell items on Amazon for more. Crazy, right?! You can sell for with a basic account without a monthly fee, but you'll probably want to upgrade to a store after a month or two.
You may not need this right away, but you'll want it soon. As a business, you'll be expected to pay state taxes each month and TaxJar literally tracks everything for you. All you have to do on State tax day is log in, click one button, and all the information you'll need is right there. It makes is SO MUCH easier than trying to keep up with it yourself and plans start for as little as $19 a month. Trust me, it's well worth it. Not only that, but you'll get 10% off your whole first year by using the link above!
Amazon Prime isn't free, obviously, but it is a really helpful service to have access to if you can afford it. I order office items, electronics, and other business-related things all the time. In fact, it's odd if I go more than four or five days without ordering something through Prime.