Being a retailer just means that you sell to the general public. You can retail your own brand that you make with wholesale blanks. Or you can retail someone else's brand.
Yes, it's legal to put your own tags in wholesale shirts that you purchase, and you'd still be a clothing brand. What makes you a brand is that you are putting your "brand" on those wholesale shirts.
Do some reading and research in this section of the forums:
T-Shirt Tag Relabeling and Finishing - T-Shirt Forums
There's lots of good info there on what you asked about and more.
I thought about going the custom made route a while back myself. In the end, I figured it just wasn't worth it (time and money) to get the kind of quality that I could already get in a wholesale blank.