I believe many people do this with a hobby they already are involved in. If you are buying and selling sportscards (for example) as a hobby, it's pretty easy to decide to make it a business. Just start keeping records, get some business cards, etc. etc. It might not cost very much to make that leap.
Now look at a PC business on the other hand. You are starting with nothing. You have more of a burden to show you have a profit motive - advertising, making sales calls, etc. That takes a lot of time and money, while sportscards is as easy as going to card shows on the weekends. If you don't sell anything, you are still out the money. Sportscards people don't care so much, they are still interested in it as a hobby anyway.
And all that for some deductions which, as I mentioned, only save you the tax on the money. It's not a free ride. And if you do it on a small scale (let's say you don't do sales calls, you just put an ad in the local paper to keep costs down), you aren't going to make a lot of money, and you have the same amount of aggravation as you would if you were wildly successful.
Look at your tax return for last year and figure out what your tax bracket is. Let's say it's 28%. Now, to take an extreme example:
Costs of running the business: Advertising, brochures, mileage, postage, etc: $500
Total sales: $8000
Gross profit on sales: $800
Net profit after deducting costs: $300
Tax on profit: $84
Net profit (the bottom line): $216
Tax savings because of deductions: $140 (if your tax bracket is less than 28%, this number will be smaller)
You earned $216 and saved another $140 because of your deductions. Estimate how many hours you would have had to put in (remember, you have to convince the IRS you have a profit motive, so you can't be a total slacker) and figure out how much an hour you would have made.
You can play with the numbers if you think you'd make more than a 10% gross profit, or you would sell more than $8000, or whatever.
I was a co-owner of a small business for a few years but I didn't have enough time to invest, and my partner and I agreed I'd put up the money and she'd do most of the work. Yes, I got to deduct the losses, but we still lost money. Not a lot, but deductions don't make up for your out-of-pocket losses.