Which Ruger Mark is this?

Looks like a MkII from the pic. MkIIIs have a loaded chamber indicator.

NOT a mkIII

In addition to markings on the side to check, the LCI, internal lock, and button release are all mkIII items. That is probably a mkII, possibly a mkI or before.

Ask the owner about field stripping, if he recoils in horror or tells you a tall tale about "these are too hard to strip" or "these don't need to be cleaned" reduce price by $100.

It's a MK II, not a MK I....MK I's do not have a slide lock back lever and do not have the ground down flats on the end of the receiver where you grasp the bolt to pull it back. Here's my MK I.

Found the chart I couldn't earlier:
http://guntalk-online.com/forum/viewtopic.php?t=934

Keep in mind that a lot of those are customized to some degree, while you appear to be looking at a very basic "standard" model with fixed sights, short tapered barrel, etc etc.

I'll say not a Mark III also. Mark III has the mag release button next to the trigger. Mag release on that gun in the picture is the very bottom of the magazine well. I have a feeling when you get it in your hands and look you will find RUGER Mark II marked on it. Possibly on the receiver above the grip. The Mark II's and III's I looked at the gun show were marked there. I also believe the Mark III's have a ten round magazine over the 9 round on the older Mark's. I am no expert so take all my words with a grain of salt and wash it down with lots of water. Good luck on the trade Ruger did good with that .22
Mike

WOW, I like that chart. I would say it has to be a Mark II

The original MK I mags only held 9....However, you can use MK II mags in a MK I gun. It is a direct fit with later MK I guns. On earlier guns, you simply move the follower button to the opposite side. If your order a new mag from Ruger, it will come with directions on how to make the swap.