Should I accept this fantasy baseball trade?

Yes because Zimmerman is on the Nationals. I think that Dunn will be traded and there will be no protection for Zimmerman. Also, V-Mart will score more runs with Grady Sizemore coming off the DL. Grady will make V-Mart better, and Dunn leaving will make Zimmerman not do so good. Besides, you already have Evan Longoria.