It's the cost of your time to clean it. If the offer is good, and you do not like the cleaning activity more than the end result (like me), and time to clean will cost more than the extra price you could get if cleaned, sell it. Otherwise clean it. The patina looks solid, but coin looks quite worn and shows scars of the already done mechanical cleaning, so +1 reason for selling it.
The decision whether or not to continue depends on just how good the offer for it is. Judging from the exposed portion it looks like with some TLC it will clean up relatively nicely, however with the reverse completely caked and covered we have a "Schrödinger's Coin" paradox going on here where at the moment the reverse is both simultaneously a common pitiful mess and a rare or particularly well preserved strike, so the offer better be at least a healthy average of the 2 values! If the offer isn't anything special I'd very happily just keep it and clean it myself, as cleaning nice coins is quite stimulating and enjoyable. Nice find!
Sort of hard to determine how it will end up after cleaning so maybe just sell it if the offer is acceptable.