It is obvious that articles are written by someone who is not a native writer as the flow and sentence structure can easily prove that. However, you are still pretty good for a non-native. You can improve a lot by writing consistently, reading, engaging in conversations in places like Quora or BHW... Just write and read a lot and you will surely improve.
I also encourage you to test and practice your grammar skills and expand your vocabulary by learning new words. When typing, try to search for synonyms by using Thesaurus.
I would not buy this kind of content to be perfectly honest. When it comes to my own websites, I try to publish authority content written and edited perfectly. If I was thinking of hiring you for my content writing business, I would refuse as I can't afford to send mid-quality content to my clients.
If you hire an editor and work beside him, you can charge your content at $1 or $1.5 per 100 words. Though, I still advise on improving as grammar is not the essence of writing well and even perfectly edited article might not be engaging, authoritative or insightful in any way.