I clean my chips, but some like the 'natural' and historical look and feel. It does bring out certain molds, etc. I do not mind getting a dirty chip - at least I know how I clean. Not sure if a novice seller might make bad choices about cleanser, and might scuff it up, leech out color, or damage a hotstamp. If I have bought it live, I have been able to judge whether just dirt, or something more permanent. If on ebay, it is a bit more chancy, but a decent scan will enable a pretty good estimate. Sometimes a dirty chip and poor scan can suppress the bidding, and I have got some nice chips at very good prices that way. THe nice Fremont $1 below in similar shape is typically $100-$130 on ebay, sometimes a bit more or less. I bid on it as a dirty chip and got it for about $80. Cleaned up really nice with a cut-down toothbrush and mild soapy water, and a fast rinse.

Just one person's take.