JohnRoy - 1:53 am on Jun 16, 2009 (gmt 0)

Nice replies so far, and agree with most.

However, OP already has lowest pricing in his market, and the question was:

has anyone who has tried it actually seen an increase in sales? Does it make any difference at all?

Answer to both: YES.

"we will not be undersold" guarantee does work to make 'em click the holy buy button.

But, try it yourself, either by running two separate campaigns (one with guarantee and one without), or run campaign for a while and compare sales vs. traffic.