is it frue if you but something for a dollar then sell it for two dollars you can make a iot of. dollars?

There are other costs to doing business than just the price of the goods. Generally, doubling the price will provide enough to cover the other expenses, but you have to sell a lot of them. How about 2000 a week? Can you do it?

If you pick a selling and popular item, than yes. If not, you will be out a lot of money and will have lots of useless junk. Many people become rich this way...

If you buy in bulk, then yes you can. But a lot of people lose money this way, so you have to be very smart.