In essence, savings bonds fall into the category of "zero coupon" bonds. That is where you pay X$ (let's say $500) for a bond which will be worth X$ +I$ (let's say $1,000) in a number of years. In the interim, you don't collect interest, it just accumulates. You may pay $30 for a savings bond with a $50 face value, and in 20 years (or whenever) it can be redeemed for $50, the full face amount.

Savings bonds are not listed investments, therefore not 'sold'.

You would usually invest ￡5000, whatever, in return for 2.5% annual interest and you get your ￡5000 back at the end of 5 years (plus, maybe, a bonus.

Traded corporate bonds or government bonds may be sold at a discount or premium depending on their coupon. The yield of a bond has to correlate with general interest rates and the only way this can happen is if the price varies.

Yes, if you mean a discount to the face amount.