Okay, so you have some great shirt ideas, or whatever, and you want to sell it. What are some great ways to sell it?
[For the purposes of this post, "shirt idea" means any kind of swag that is related to your comic. If you are selling Frisbees with character heads on them or key chains with logos on them, it counts as a "shirt idea"; this just saves me time when it comes to writing, so go with it.]
Print On Demand: This comes up a lot in this blog, but here we go again. Print on demand allows you to create a wide variety of shirt ideas, but not have to print off huge numbers of them. You make the design, upload it, and then decide what kind of shirts it looks best on. Each one has something that makes it unique, such as a wide variety of potential merchandise (CafePress), buyer personalization (Zazzle), or the ability to put the design into the general pool (Spreadshirt). If you like the way the design is put on the shirt, then go for it, but there are a few different ways to do it. For those looking for something entirely different, try Wodans.
Auction Sites: Once you have your shirt idea done up, now you need to sell it. This is where sites like Ebay, Bonanza and Amazon come in. What you need to do is develop a dropshipping system; in essence, you sell something, and then use the funds generated to buy the item and mail it to the person. There are a number of problems with this, and it is best for when you are starting out and can't afford to print off a number of shirts; when you can afford to, print and ship your own shirts. But this is the best way to do it. Word of advice: Make sure you have a bank account or actual credit card (PayPal card doesn't count, people!) or you may not be able to play.
Mini-stores: Some POD sites let you set up mini-stores. This is a widget that allows you to sell stuff straight from your comic without having to go to an auction site. This cuts out the middleman, means you don't have to worry about shipping, and greatly simplifies your life. Just a consideration.
Shirt Sites: There are number of sites, such as Threadless, that do nothing but sell shirts. Sometimes you need to qualify to for the site, such as Threadless' competitions, but they can be worth it. There are number of sites out there, and some of them even support webcomics, so it may worth checking them out.
So, if you are looking for a way to get paid for your artistic vision, this may be the way you do it...