You can make whatever choice you want. It's not up to me what's illegal or not (thats up to the lawyers).
I will explain why its risky, so you don't think im just being a contrarian.
The reason it says in giant capital letters "NOT FOR HUMAN CONSUMPTION":
The simple explanation is that 99.8% purity might sound good on paper but it isn't acceptable for human use.
The more accurate explaination is that the final-step manufacturer will control purity and contaminants because they are legally liable to do so. There are also important things to control beyond purity. For example, the dosing mechanism for an injectable or oral system delivers an extremly precise, controlled dose. You cant get that with DIY injection or mixing at home. So your delivered dose is going to vary in a way that is unacceptable.
See 21 CFR part 211:
www.ecfr.gov
I'll spare you the math, but a much higher level of product quality is required by the FDA before selling a drug to the user. A quality system is expected to deliver a proveable 99.99966% "known good" acceptance rate for a finished injectable product, and like I mentioned, it accounts for many more potential quality problems beyond drug purity.
If all that sounds expensive, thats because it is.
penguinpeptides.com
