Can a salesman sell everything even if he is not convinced?

If you are not confident about the product or service you sell, it is going to be a tough for the salesman.

Ofcourse Not, he must be convinced in product first. 

A Sales Professional can only sell/market if he is convinced about it from all aspects i.e.
specifications, quality, brand name etc.
If he does not believe in what he is selling the lack of conviction will somehow seep through to the consumer.

You can sell only if you yourself are convinced: If you are not sold on the product or service, it will be an uphill battle to sell someone on else.
Your lack of conviction will scream through.

confidence and enthusiasm is key to any sale and some understanding of the product.
It is very important to like and believe in your product