I'll never forget the time I went to a garage sale and bought a handful of t-shirts to make a rug with only to find out that the material when cut into strips frayed and made a huge mess.
Here is what I found out. The pictures are a little hard to see, but hopefully you can tell what I'm talking about.
If the material looks like stockinette on the front and inside, it will not work. At least it did not for me and the way I cut mine into strips.
However, if the material looks like stockinette on the front and garter stitch on the inside, it will work.
Hopefully this saves you any hassle from buying the wrong type of shirts.