An LGS has a Model 18-3 that appears to be in about 90% condition finish-wise, very little turn line, seems to lock up tight. What concerns me is there are peening marks between each chamber on the cylinder face. What does this mean? How much of a cause for concern is it? If the gun shoots alright now do I need to be worried about it? They're asking $599, which doesn't seem too bad considering how few and far between these are.

Pass on it, especially at that price.
"Peening" marks between chambers is NOT a good sign.

If the marks are on the front of the cylinder, the gun could have so much end shake the cylinder is slamming into the rear of the barrel.

If the marks are on the rear of the cylinder, it sounds like the gun has a serious timing problem and the hammer has been dropping between chambers with the firing pin striking the cylinder. This probably also damaged the firing pin.

I thought of that, but the hammer's falling at the correct point - firing pin is aligned with the chamber. I suppose it was out of alignment at one time to cause this - if its been fixed, do I still need to worry about it, or is it likely to recur? Oh, meant to say, the peening's on the rear of the cylinder (firing pin side). There's a range right at the dealer's; if they let me try it before I finalize the sale and it's OK, am I in the clear?

Is it truly peened or are you mistaking burn marks and/or carbon and lead build up?

I would pass on it. Not worth the chance.

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Pictures or confusion.

I expect that 300 dollars will fund having a new cylinder and firing pin fitted, so the real question is do YOU think this revolver is worth 900 dollars? I think we both know the answer to that question. If that gunshop will knock it down to 400 bucks I'd say buy it, otherwise I would pass on it. Because for about 800 dollars you can purchase a brand new completely unmolested model 18.

It might function perfectly when fired SA and DA in what most would consider a normal action speed.
Sometimes the cylinder will skip by the locking notch and not lock into position when the action is functioned,, very briskly,,for lack of a better term.

That will leave the cylinder face in a random position to receive a firing pin strike,,the 'peen' mark(s) you see.

It's more often a problem on the large N frames with their larger amount of rotational momentum to carry passed the lock up point. But any of them can exhibit the problem.
A weak cylinder stop spring is a good place to look first along with a cylinder stop itself that may have been polished and/or altered to 'improve' it's function.

I had a Model 35 a while back that had been peening the rear of the cylinder quite badly. Even the small J frames can be victims of it.
Simple functioning fix on the 35 was the cyl stop spring. Fixing the peen marks took more work.

Checking for the problem is a problem in itself since it requires you to do some dry 'speed' firing to get the revolver to display the problem.
Emptys or snap caps in the chambers help the with the proper function,,but if/when it does skip, you'll put another peen mark into the cylinder face.
If it's not your gun to begin with,,the test and the results are probably something the seller isn't going to be happy with.

Personally, unless the cause of the problem is visually apparent and will also lower the price accordingly,,I'd walk away from the pistol.
With that little external wear and use showing on the gun, I'd suspect someone has 'tuned' the action and the peening is one of the results of their efforts.
JMHO

Thanks for the advice. I've just been told another MD shop has a good pre-17 K-22 for another $100, and I'd rather have that than an 18, anyway.