Where there's muck, there's brass!
Do you know what most antique gun dealers do with an antique gun when they get it? They do nothing, other than maybe wipe it down with a soft cloth.
Do you know what museum curators or consevators do with an antique gun when they get it? They do nothing, except maybe examine it to see if there is a problem with it decaying somewhere that needs immediate help stop it going further. Other than that, the leave it the way it was found.
Old dirt is a sign of age. Clean it and you have removed evidence. Clean it spiffy and remove dirt, patina and redo the oil finish, you have removed forensics that prove it to be old.
Counterfeit antiques have the signs of age faked and added. Not always can the faker remove signs of newness. Dirt, grim, rust and dust can all be added to fool the buyer.
Now I do realise that we are not dealing with an antique. Nor are we dealing with something extremely valuable. But the same concepts with regard to preservation and cleaning seem to apply to mil spec rifles these days. They are indeed old and collectable. However, I would clean a 200 year old Brown Bess musket a little differently than a ten year old Chinese SKS.
A genuinely old and dirty rifle as opposed to dirtied up one is quite distinctive when you have handled both.
Khyber Pass rifles are smothered in god knows what kind of shlit from goats, camels, fakers themselves, who knows. As I understand, urine is used in the aging process amongst other ingredients. I always wash my hands after touching one of those.
Nepal cache rifles were coated with Rangoon axle grease and stored in a decaying building right next door to a brick factory. The brick dust mixed with grease and allowed to mellow for 100 plus years is nasty.
The OP asks about a grimy rifle and what gives with that? I myself would prefer the dirt still on a gun, then I could clean it myself properly. If it is old dirt, or if it is mucked up to hide something or to fool somebody, I think that I could tell.
I bought a very nice Lee Metford cavalry carbine from a member here. Total time capsule. Dirty and gross. Every nook and cranny has old grease and dirt in it. Some globs of congealed something or other in crevices. But, it is genuinely old, the dirt fills the screw slots and the gaps between wood and metal. This carbine has never been touched since before I was born. No hunter or amateur gunsmith or hobbiest has done so much as turn a screw on it . I paid a premium for it, because of its untouched state. Man, I have to tell ya, that the urge to pull her apart and get rid of the dirt is incredible, but I have resisted.
All I did was to wipe it down daily with raw linseed oil on a rag and then gently buff it with soft cotton flannel cloth. The crud on the woodwork (which is usually a mixture of oil, sweat and human skin cells) softened and lifted away after a week or so to reveal the patinated wood underneath. The metal remains black, but now has a satin semi gloss instead of blotchy and crusty flat black. Even with such a gentle cleaning, I feel that I might have gone too far, I should have left it untouched.
So buying a dirty rifle? For me, no problem if the bore is clean. If I sell one, I consider it simple courtesy to at least bore snake it for inspection by the new owner. Shooting and leaving the rifle dirty is an old trick to hide some problem with the bore. Dirty outside, no probs. Dirty bore to me means red flags going up.
More than once I have bought a rifle where the seller admits that the bore is a sewer pipe, price adjusted accordingly. Then when I get it home and clean it, suprise! Amazing what a bore snake can do with just a couple of passes.
But to tell the truth, old dirty guns catch my interest over a bright shiny ones.
The hobby of restoring has become popular enough now, that I find the need to keep my eyes keen for signs that something might have been restored. Could indeed be an official FTR in some cases, but more often or not, something desportered post service. Part of my hobby is restocking or redoing what someone else has done badly. With a Lee Enfield if the forearm is not fitted correctly, fitted tight where it needs to be fitted tight, accuracy will suffer.
Bubba not only sporters rifles, he thinks that he can restore them too. I mean, a full stocked army rifle is worth more than a sporter, right?
Perhaps half of the Lee Enfields on the EE have been tinkered with, but that is just my guess without handling them.
The subject rifle would get a second look from me.