I will be honest here, I have a love/hate relationships with auctions. They are expensive for the short time span they are live, you don't always make much money but you may hit it lucky with these, you can really widen your customer base quickly and they are pretty exciting.
Online selling of artwork really took off with ebay - this is the where a lot of artists developed their careers independent of being represented by an agent or gallery. In the early years it was the Wild Wild West of the art world.
It is quite simple to list an auction, open an account on an auction site (I use ebay), take your quality images, write up a listing very similar to how you would in your online store, and then set your starting bid.
Now. This is the tough part... what do you do for a starting bid? There are many theories on the best way to go about this - I suggest you familiarize yourself with some auction lingo before going about this.
Starting/Opening Bid: This is the first bid price. If you list your item for $0.99 hoping for it to get bid up to a higher price you still take the chance that it will get one bid and sell for $0.99. Once it sells, it sells for that price. You cannot back out. Think carefully about your opening bid and list your item for the lowest you can go without feeling regretful if it only gets one bid.
Reserve Auction: This is an option if you don't want to take risks, you can set a reserve on a listing and have a low opening bid to attract buyers. If you put a reserve in place, it remains secret and the item only sells if it is bid up to that amount.
The pros to this is that lots of action on an item means more interest from buyers - they like to feel confident that others like your work as much as they do. The cons - if you set your reserve too high, your ultimate goal (selling your work) may not happen... this means you may pay more listing fees to move that item to another venue or relist it.
Listing Upgrades: these are available to you in a listing, sometimes things are automatically checked off (uncheck them if you don't need them) You can choose a longer listing (10 days over 7 days), Highlighted listings, Feature listings, Automatic Timer (to list at a pre set time) Extra pictures, there are tons and tons of things that you can add.
However, on ebay, if you are going to add things to your listing, you are going to pay for every thing you add. a dime here and there doesn't seem like much but just be aware that a listing can get pricey pretty quick. There is a reason why ebay earned the nick name "feebay" - you can take a $0.55 listing and make it over $20 per listing - which is only live for a week!!!
There can be advantages to you to using some of those features - they may bring up your viewers immensely, occasionally trying something out is good to do if you have a good strategy. Make sure you have a bunch of cheaper auctions live at the same time if you do a high priced auction - buyers will check out your other work if they like your featured auction. Get the greatest bang for your buck!
Insertion Fees: Always be aware of the basic price your listing should be, it is based upon your starting price - the lower your starting bid, the lower your listing fee. Only use the extra features that you want - if there are automatically checked in boxes, be sure to uncheck them if they are not something you want to use.
Take advantage of 5 and 10 cent listing days - the notification on those shows up in your message box a day before they occur. Just be aware that every other seller gets the same message so the competition goes up considerably as they take advantage of the deals too.
Final Value Fees: There is a percentage that ebay will take for the sale of your item on top of the insertion fee. Keep this percentage in mind - read up and be familiar with what it is and remember if you are accepting paypal, this will also be a percentage taken from your sale price. You may want to build a bit of cushion into your opening bid to account for that and prevent regret.
Now, if you are suddenly turned off by all the fees, keep in mind that if you were selling through a gallery they would take a 30% - 60% percentage and online 7% - 10% isn't too bad compared to that. Just be aware and adjust accordingly.
Shill Bidding: DO NOT encourage friends or family who want to "help you out" by bidding on your auctions with no real intent to purchase. False inflation of your prices is not only dishonest, it can result in your auctions being pulled and your ebay account closed.
Also, what sense of accomplishment can you honestly gain in the long term if the only reason your stuff is selling for higher amounts is because people take advantage of a generous bidder. It is better to set your opening bid higher and see what happens than have a relative in cahoots with you.
Anyways, there are more answers to common selling questions on ebay - check out Seller Central or do a search on key terms. When you are looking at their suggestions always keep it at the back of your mind that ebay makes its money off of YOU. They are going to tell you how wonderful all their products and services are so that they can sell them to you.
Start out with basic listings as a base line and then add things here and there one at a time so you can see for yourself how they affect your sales. Good luck!