Answering your questions:
1-Does it count as academic fraud? Has any University taken action against students who have used online proofreading websites in the past?
No. fraud is intentional deception to secure unlawful gain. Proofreading is not a problem. In fact, many universities and event governments demand professional translation and proofreading for important papers and/or tramits if they are in a language different than the official one. The thing is that proofreading is done after you already wrote your work, so it is still your work. The problem is when you use a service to write the work itself.
2-Can the seller use my essay to write other essays? If yes, will it lead to plagiarism
Yes and no. Yes because they can indeed grab the text and paste it in a new doc, which would be indeed plagiarism on their part and fault.If you publish it first then you are protected by author's rights. And No because an essay has a personal way of writing and it's on a very specific topic, which makes it hard to use directly without it being obvious.
3-Would you personally recommend using Fiverr?
Yes. If you are just proofreading. But do check the qualifications of the people offering the service first. Being native to a language does not mean people are aware of the language and it's rule, which is specially common for monolinguals. Also check with some editorial agency to contrast prices, not to mention that any of these agencies can give you written proof that it isnt/wont be used for other works. I understand the price for this is a concern, more so for a mere essay, so I can suggest you check a language teacher/school. They can proofread too, possible at a a decent price, and they will understand better what you meant to say in your original language.