Is there a specific amount of money at witch I don't need a tax ID to run a online business?

You always need a tax id to run a business regardless of income. if you dont have one as a corporation (the number format is xx-xxxxxxx) you must use your SSN. try as you might, there is no way around those pesky taxes. if you run an ebay business, the irs started looking for power sellers and running numbers to see who is getting what and you can expect a knock at your door sometime for an audit. and ebay will not shield you by refusing to give the government records when they ask. it is just smarter to do things by the book with the irs so they don't come back and make your life miserable when they catch you with your hands in the cookie jar.

that being said, a company is required to get a w9 from anybody they do business with, which must show the name of the person/company they're doign business with, address and either TIN or SSN in the case of an individual. if any company you do business with gets audited and the irs finds they're taking a deduction as business expense any monies they send to you without having a current w9 on file, that expense would be dissallowed and somebody would again be knocking on your door. they might issue a 1099 as an independent contractor in which case you are still on the hook come tax time. that is unless your business with them is less than 600 bucks for the year, then it is up to you to declare that to the irs yourself.

You already have one, your SSN. You may get an EIN if you wish if you are a Sole Proprietor. If you form a corp or partnership you will need one.