Well to start, I’m sorry that happened to you. Those are the kinds of buyers who make the honest ones look bad.
Second, I’d say in the finished product, if you see random noticeable mistakes or symbols that shouldn’t normally be there, then double check it. Because simple sentences that have a ton of mistakes are most likely done on purpose to get past plagiarism checkers. As for knowing before you buy?.. Request samples of their work, or make sure you have a conversation with them beforehand? Check with them on how their availability is, ask them to Include sources of where they found their information?
As an honest writer, those things are simple tasks to do. (I am a writer, even though it’s hurtful when buyers second guess my work, I have to try and remember it’s most likely because they’ve had experiences like you)
Again, I’m sorry that happened! I hope this helped! Haha