Does that mean any gains I make from selling things I own is tax free up until I make a total of £10k? Is this only if I don't have a job already?
In broad terms, yes. But what you sell has to be a capital asset. And if it is a chattel (tangible moveable property) then special rules apply. The £10,600 annual exempt amount applies to all ... whether employed or not, whether earning £0, £10,000 or £10M.
Does this mean if I was to buy a car for £2000 and sell it for £2500 that £500 gain would be tax free? If I buy 20 cars for £2000 over the year and sell them all for £2500 each making £10000 would that be my money in my bank tax free?
Cars are specifically exempt from the capital gains tax regime. In simple terms this is because they are usually depreciating assets and this legislation prevents people from claiming capital losses on cars (and other machinery). The situation that you describe, with a series of purchases/sales and a profit motive, is typical of trading activity. Such profits are assessable as income, not within the capital gains regime.
I am sure this cannot be right
You are correct, it isn't right - but there is no problem in asking and there is no such thing as a stupid question. You have come to the right place.