Value of Bonds // Why do bonds sell under the same method but different prices?

Need this question answered, need at least 350 word answer.

If all investors are using the same method to value bonds, why does the same bond buy or sell at different prices? In other words, why is there a market for bonds? Why do some financial analysts treat preferred stock as a special type of bond rather than as an equity security? Include in your discussion the relationship between bond prices and interest rates.