As to the array of goods: eBay sells just about everything all over the world; Etsy sells mainly handmade crafts or their supplies and vintage items; Amazon sells a wide array of goods but you can only sell in Amazon Market Place as an alternative to a good Amazon already has listed for sale (thus requiring no pictures from you as it already has the item listed with picture).
Buying Basically the only things you need to buy on any of these entities is a source of payment and a street address for delivery. The safest form of payment to use is to set up a PayPal account for using Etsy and eBay. This is tied into your checking account and/or a credit/debit card. For Amazon you need to put a credit or debit card on file. One huge advantage to using PayPal is that whenever I have wanted my money back, I just file a complaint with PayPal, PayPal investigates and thereafter I typically get my money back. The most frequent reason for this is not getting the item or getting a intrinsically flawed item. This happens very infrequently and at no greater rate than it would with any store one uses in the real world. A big bonus has taken place in recent years: now a huge amount of other stores all over the online world also accept PayPal so I rarely have to leave my credit/debit card info anywhere else.
Are the deals good for a buyer? Yes. With just a few exceptions, you can get excellent discounts on most goods. My number one exception to this is electronics. My best electronic buying experiences have been using either the Apple store online or getting into my car and driving to a store and buying the item in person. This covers everything electronic, from tv sets to computers to mobile devices. That is now the only way I buy electronics. I have better luck, for example, going to Walmart in person and buying electronics there than I do buying any electronic item online, except at Apple.
I have recently reached that same point with appliances. I have had such bad luck buying appliances online that this category as well is now in the get in my car and drive there category. I have otherwise bought, at huge discounts, clothing for Jim and me, dishes, art work, art supplies, including fabric and beads, and a wide array of other consumer goods at all three of these etailers. Some of the very best deals are if you are willing to buy outside the USA, from say Thailand or China directly. Any such seller who is on American ebay files everything in English so there is no language barrier and no currency barrier because of PayPal.
Selling: I've sold at all of these and the main way this is profitable is if you have a brand name item which is no longer for sale. Thus, I sold very quickly and at very good prices: Lenox dishes which Lenox no longer sold; a Mosaic jazz recording set which collectors want which is no longer in retail existence; clothing items from designers which would be considered desirable vintage (this actually resells quite well) such as Missoni and Miyake; high quality vintage musical instruments, etc.,. The list goes on but these are things which are easy to sell because lots of people are looking for them. They even set up search alerts so as to be informed instantly when such an item comes on the market. Selling becomes a lot harder and less profitable with most other items.
One also needed, once upon a time, a lot of computer skills in order to sell goods at any of these venues. However, this has become easier and easier so that today the main skill one needs is the ability to take good digital pictures of one's goods. I've taken such pictures with just the camera on an iPhone or iPad so no separate camera is needed.
How To: Your best bet for learning how to sell is by viewing the videos at Lynda. It also has buying videos but buying is a lot simpler. However, if you want to take every safety precaution for buying, then go view those videos too. There are tons of books for sale on using eBay, Etsy and/or Amazon. Amazon carries all of them but so does your OverDrive Library app for free. Libraries have been very good about stocking these ebooks for online library use. These books all have their merits but my number one test is that the book needs to be as current as possible. These sites change so rapidly that materials which are dated by even a few years are hopelessly obsolete.