Luckily the caps i have received in the mail that had blemishes weren't beat up on the outside. Key cap puller dents and chips are the most common issues i have run into (and probably a lot of other people to)
Try one of these guys out. Four-8 sided fingernail buffer, using only the shine and buffer sides, but tread lightly if you want to use the more abrasive side.
Hope that helps, you'd be surprised how much a light buffer can bring an old cap to life after a nice wash.