While this will mostly apply to those who sew or make other handmade items, the basics can apply to most every online seller (I say most because there's always an exception).
First, know how the platform you're selling on works.
eBay has changed a lot from when I first started selling there, and I'll admit I'm not up on all of their rules now since I rarely use them anymore. eBay does provide a very well known name, and most people think to check there first. If you do want to sell there, be aware of their fees. They do provide 50-100 free listings each month, with some exceptions, but their final value fees are fairly high. They also charge different fees for different categories. The final value fee will be 10% of the item's value, up to a cap of $750.
You can view the entire breakdown of eBay's fees here: eBay Fees
Titling on eBay, you want to make sure you use good, descriptive words that will accurately tell what your item is. Just titling something as a "brown shirt" isn't going to get you good search results. You want to make sure you're going to get as high up in the search results as possible. Using "Mens Lg Large Brown T-Shirt Cotton New Designer" is going to get you much more attention.
Etsy is constantly growing and have started making more improvements lately. Etsy is also limited in what's allowed to be sold there. Only handmade items, vintage (20+ years old) and supplies are allowed. Etsy doesn't get as big of an audience as eBay, but it is expanding and drawing in new attention weekly.
Etsy's fees are considerably cheaper than eBay's. They only offer 40 free listings if you refer a friend who then opens a shop. But otherwise, they charge a flat .20 listing fee for any item for four months. eBay's regular listings run from 1-10 days for comparison. Now, if you list multiples of an item (say a quantity of 10), and someone buys 5 of them, you will be charged .20 for each item, so your bill will reflect five additional .20 charges as renewal fees (four for the additional items the person bought and one to relist the remaining quantity). You are also only charged 3.5% of the item's value as a transaction fee.
On Etsy, buyers are allowed to pay in a variety of ways. Few sellers accept checks or money orders, so most only use Paypal and Etsy's Direct Checkout. Etsy's Direct Checkout allows buyers to pay with a credit or debit card through Etsy. The fees for direct checkout vary by country, but in the US it's .25 + 3% of the total paid.
You can view Etsy's Direct Checkout fees here: Etsy Direct Checkout Fees
And Etsy's seller's fees breakdown here: Etsy Seller Fees
Etsy uses a system of titles and tags for search. They also have their items in Google Product Listing Ads, so your titles are very important to be found there. The tags are how Etsy finds your items in searches on the site.
Back to our brown shirt. While on eBay you were good to just list the size, material and color, on Etsy, you need to go bigger. You need to think like a buyer, and use words in your titles and short phrases in your tags that a buyer would use when looking for a shirt like that. So your title on Etsy could read "Mens Unisex Lg Large Brown Dressy T-shirt New Cotton Designer High End Heavy Work". That's going to get you picked up if someone does a search for "mens large t-shirt", "heavy cotton shirt", "cotton work shirt", etc. Then in your 13 tags, you want to use different phrases taken from your title (limit of 20 characters per tag). So you could use tags like "cotton shirt", "brown work shirt", "brown t-shirt", "designer t-shirt", "large t-shirt", etc.
Also, while eBay pretty much has a built-in audience of buyers, on Etsy you need to do all of your own promotion work. It's very much like having your own brick and mortar store. Some common places to promote are Facebook, Twitter, Instagram, Pinterest and handing out business cards if you sell at farmer's markets or craft fairs.
Your Own Website
This one allows the most freedom, but also requires the most work. You can design your site however you want and use any number of online shopping carts. Paypal offers a shopping cart to use on your own site and the setup is fairly easy and painless (that's what I use on gypsy-red.com).
While you can design your site any way you want, and there are a number of places like Wix that make it pretty much point and click, maintaining your own site requires dedication and constantly looking to improve it and expand the offerings. It also takes a lot of effort to promote it and get traffic there. You also need to get very familiar with meta tags so you can be sure your site will show up in search engines.
There are a number of other options out there for selling online. Ruby Lane, Shopify, Storenvy and more are growing in popularity. Spend time on their forums where possible, read help articles, and figure out how their search functions are set up and will apply to what you're selling. It takes a lot of work to build a successful online business, and I know a lot of sellers who work 18 hour days on their businesses. But stick with it, be willing to constantly learn and adapt, research, research, research to find the right venue for selling your items, and you can make your business successful too.