Costs to produce/purchase the shirts you want to sell.
Your markup, profit margin and retail price.
Knowing your target market.
Being able to cost efficiently reach your market.
Being able to effectively drive traffic to your site.
Convert that traffic into sales.
Just a couple things I'd add to that is the web-based costs...
Consider into your costs the monthly/yearly domain fees. You'll likely pay a fee for some type of "checkout" tool on your site (if anyone knows of free services please suggest them) especially if you want people to be able to pay by credit card.
Domains can be relatively cheap, but things begin to add up on the "web" side of it when you want to add more and more and be able to sell items from your site (from what I'm finding in my research)
Web based labor and costs will be a big factor. It will really pay to do search engine optimization...though it is a slow painful process. You can do a lot of the work yourself---just do your keyword research.
You want to show up in the organic searches...I am sure "T-Shirt" is locked down pretty hard...but if you come up with niche words, it will be much easier.
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