I see that it's possible to start an auction at any price but you can add a reserve price below which you wont sell. Now when my reserve price is 150$, what is the idea behind letting the auction start at $7 dollar?
It's just the nature of auctions. If the seller wants, they can set the starting bid to be their reserve price.
That is how I know offline actions: starting bid = reserve price. But here I have the option to let people start bidding at a price I am not going to sell it for. My question is then: why would people bother to bid prices I am not going to accept? What is the benefit for a seller to let people start at any price when he has set a reserve price?
Because when they start the bidding lower it garners more interest and attracts those early bids. When people see others bidding, they'll see there's actual interest in the product / site / etc. and bid as well. It's just a marketing strategy to attract more eyes.