Anything you do as an "ongoing trade or business" where you have a clear profit motive and take active steps to make money, is considered self-employment. You are required to keep accurate business records of your income and expenses, and report your income and expenses on schedule C. You pay income tax plus an extra 15% self-employment tax.
There is no such thing as "writing off" your purchases!
You can deduct ordinary and necessary expenses of conducting your business. Which expenses are ordinary and necessary depends on the nature of the business, and you must further separate "expenses" and "assets".
Assets are things you buy that have an expected life of more than one year. A computer or tablet would be a business asset. Assets are normally depreciated -- roughly speaking, if you buy a computer with an expected lifetime of 5 years, you deduct 1/5 the cost each year over 5 years that you use it in your business. If you convert it to personal use, you stop taking the depreciation. There are some special rules where you can deduct all of an asset's cost in 1 year, but that has further tax complications. For example, suppose you buy a computer and list it as a business asset, and one of the special rules applies and you deduct the entire cost. Then, you convert it to personal use. You have to repay most of the deduction. Or, you sell it to someone else as a used computer. If you deducted the purchase price as an expense, then the selling price you receive is business income.
There are no free lunches, you can't buy a computer, use it for a month and rate it, and then sell it or give it away, and keep the full deduction.
This also applies computer you use to edit and upload videos, as well as your video equipment. You can list it as a business asset, but when you are no longer using the equipment for business (because you quit the business, or replace the equipment with newer) that usually creates a taxable event.
Now regarding smaller items like supplies, you can't deduct personal items as business expenses. I don't know what "stationery and such" means -- let's say fancy pens. If you buy a pen, make your video and rate it, and then keep it for personal use, that's not a business expense, it's a personal expense. If you sell it on eBay, you can expense the pen but the selling price is taxable income to the business. It's only really a "write off" if it is used up and thrown away.
Furthermore, if you get "free" items from vendors, review them, and then keep them for personal use, that's also taxable income to you, at the fair market value of the item.
Some people certainly make money the way you describe, but you can also get in a world of hurt with the IRS if you don't follow the rules.
