To my knowledge there is no federal restriction on selling hi-cap mags. Some states don't want you selling them into their state from out of state if they are more than ten rounds. I believe Mass., Calif. and New Jersey have such restrictions but check with someone who knows for sure. If you sell the hi-cap mags within the state I think that's still OK, but again, check to be sure. Hi-cap mags on a P88 add substantially to the value. They are expensive.
BTW, this is the original P88, not the Compact model. right? The original is a much more desirable gun, IMO.
Bill