Personally and professionally, I wouldn't use, nor sell that subframe, especially without noting that you make no warranty and take no responsibility for damages/accidents/injury, etc.
From what I can see in your pictures, most of those welds have a great deal of lack of fusion (cold lap / overlap), and I see quite a bit of undercut. I'm not judging these welds based on any code or standard, but those discontinuities are clearly observable.
Notice how the factory welds are smooth and the edges (toes) transition into the parent material? That's what you want to shoot for. All of that cold lap and undercut poses the potential risk for failure because they are stress risers. If, say, one of those welds saw enough localized stress, there's potential for a crack to initiate, propagate, and lead to a failure. It may not be a catastrophic failure, you may not even notice it, but that isn't a risk I would be willing to take with my own cars, nor would I take that risk on a fellow enthusiast's vehicle.
I believe it important to share this information to our fellow enthusiasts to prevent any accidents, injury, or worse. I hope you see it that way too.
All of that being said, if you have questions regarding welding or welding education, please feel free to contact myself. I know there are a few more experienced members who have or still do perform fabrication work. Sometimes the internet can be extremely judgmental and opinionated, especially when it comes to welding. I'll do my best to stay unbiased and friendly.