Using the formula:
buffer capacity = (change in moles of base added)/(change in pH)

Can buffer capacity be negative, or do you always take the absolute value of the change?

By definition, a buffer solution contains either an acid together with it's conjugate base or a base with it's conjugate acid. In either case, both acid and base are present in a buffer solution.

The buffer capacity is a measure of how much acid or base can be added to the buffer solution until the original acid/base is completely reacted.

For example: If you start with an acidic buffer solution and you add base to this solution, the buffer capacity will be determined by the original amount of acid in the buffer solution. Once all of the acid reacts with the added base, the buffer capacity will be reached.

Since a buffer solution contains BOTH acids and bases, the buffer capacity will ALWAYS be a positive value.
Have a look at

http://www.chembuddy.com/?left=ph-calcul...

It gives a nice concise derivation of buffer capacity. Hope your comfortable with the differential calculus.

If you follow the derivation, the buffer capacity will always be positive.

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