Where are you getting your shirts from?
Like do you have a distributor that you use, or are you just getting your hands on shirts randomly.
As much as it may suck to take a hit right off the bat, I highly recommend setting yourself up with at least a few hundred dollars worth of stock.
If someone orders a shirt from you and you then have to order it from a distributor, then put the artwork on it, then mail it out to the buyer, you are looking at a few weeks at least before the buyer gets their product. Most buyers are fie with a bit of a wait, but trust me when I say that when you can snap off their product and have it at their door in the smallest time possible they are always ten times more impressed and more likely to buy again or spread the word about how great your business is.
Even if you just get a bunch of blank shirts and hold off until an order is placed, then apply the appropriate artwork, that saves a lot of time.
Also, what method are you using to put the artwork on your shirts?
From my experience a lot of people go with a heat transfer decal because it is cheaper, but a screened design looks better and lasts a lot longer.
I assume you are going with heat transfer because it is a much more affordable and manageable system.
I'm actually going with a well-known local screen printer to handle all of my manufacturing. They're cutting me a reasonable rate on my shirts and printing with a two week turnaround time. Artists get money up front, and then percentage royalties on sales. I personally will only be handling the business end of it - running the site, handling finances, shipping inventory, and paying contractors/outsourcing(artists and the screen printer).
I do agree that having inventory on hand is ideal, but since I'm bootstrapping with a relatively low profit margin I don't really want to sit on any inventory I don't need, and I'm really not sure what sort of initial inventory I should go for (mens vs womens shirts, sizes, etc). The current plan is to run preorders to get enough of a following that I can begin predicting, to some extent, general trends, and then begin ordering in inventory as called for.