A good trick is to thrift “by touch” - feeling for cotton or heavier materials. There are a lot of quality cotton XXL blood drive/college or work event t-shirts that no one will ever want. I’ve volunteered doing some sorting through donations for a homeless shelter before, and they weren’t lacking for those at all. It was more a lack of dress pants for job interviews that were needed.

Resellers are usually going for names and not necessarily quality from what I’ve noticed.