Triz is a double agonist while Sema is a single agonist. Generally, more agonists = better, although some people will prefer one to another due to the side effects.

Reta is a triple agonist that is basically Triz + glucagon. Bodybuilders love it because it'll repress appetite but also burn fat. As for why they're cheaper, it's because the Chinese will manufacture them for cheaper than the pharma companies, not slap a tremendous markup on them, and then sell the raw powder to the steroid sites.

The online stores that sell these usually are also reputable websites that have been selling anabolic steroids for years. There's a forum that discusses various vendors with people sending samples and posting lab results with content/purity. I don't think any of these reputable sources would then try to sell bunk weight loss drugs, because once found out, the word would quickly spread in the bodybuilding circles and people would just buy from competition instead.