Lack of cleaning has more than one aspect. Just crud build-up etc will not actually usually harm a gun mechanically, altho in some cases if enough builds up it can create reliability issues.
Other side of the coin is - is uncleaned barrel so copper fouled as to be near impossible to get clean?. Are revo chambers in .357 ringed with 38 spl fouling? - this can be real tricky after a long time.
Most important - has this neglect resulted in undesirable effects internally? That would be my major concern. Has grit got in - has moisture got in? What hidden surprises are in store. If someone is lax enough to never clean then who knows what else has been neglected? I do not clean as assiduously as some but I do stay on top of important aspects that can affect reliability.
To make a fair price for guns like this would for me require a strip and clean to evaluate before buying - then come to a fair price - all might be well and cleaning might be successful. There again, surprises may be waiting!!
The P3AT - minimalist lube to internals - basically what you can see after field strip - VERY modest touch of Mobil 1 maybe. For slide rails and frame - a good quality grease that is OK on polymer. Good ol' RIG is effective, then there is Superlube, Militec1 grease (tho intended primarily for metal/metal surfaces). These lil' guns benefit from very regular cleaning IMO to avoid problems.
In essence, don't shoot the gun totally dry and you should be good. Give feedramp a nice buffing too while stripped down.
