I have noticed the same thing on some of my RAW pictures taken with my D5000 when I open them in Capture NX2. Strangely enough NEF's from my D7000 are not affected. I am wondering if this has something to do with the in-camera settings of noise reduction or Active D-Lighting.
Check what they charge for accessories (batteries. memory cards, etc.) If the accessory prices look unreasonably high it's usually bad news and probably not a good idea to buy from these people. Check resellerratings.com and look for the bad reviews. I only buy from Adorama and B&H for new and refurbished, KEH for used equipment.
Xtreme66, like I said before you can get a used 60mm 2.8 AF-D for about 250.00 on ebay!
Here is the link to one of them!
http://www.ebay.com/itm/Nikon-Nikkor-Micro-AF-60mm-2-8-D-Macro-Lens-220235-/121029388897?pt=Camera_Lenses&hash=item1c2de9ea61
GoofyNewfie is correct! You will have trouble getting action shots but when the lens is used for macros or portraits you can focus manually with the camera mounted on a tripod. No need for VR then either. I am pretty sure even the D3100 has an electronic rangefinder and will be able to measure focus distance with the older lens. Check your camera manual on how to use manual focus. I am kinda partial to original Nikon glass so depending on how much the lens will be used I would check e-bay for a good used Nikon (AF or AF-D) macro lens which can be had for cheap. The Nikon Nikkor Micro AF 60mm 2.8 D comes to mind which can be had for about $250. With the money you save you could later upgrade to a higher end body like a D90 or D7000 which will auto-focus these older lenses.
Jono, according to the Tamron website the lens you mentioned is an older AF-D style lens for Nikon which requires an auto-focus motor in-camera. The D3100 does not have a built-in auto-focus motor so this particular lens would have to be manually focused on your camera. The lens JR1 suggested (Tamron SP 90mm F/2.8 Di Macro 1:1 VC USD) is the recently announced successor to the lens you mentioned and does have a built-in focus motor and therefore will auto-focus on your D3100. Tamron lenses with autofocus motor can be recognized ed by having the letters USD ( Ultrasonic Silent Drive)in their name.
I heard in medieval times in Merry Old England a knight would always veer his horse to the left for oncoming traffic (other knights!) so he could draw his weapon more quickly with his right hand. Not sure if that's still practical today with a car though.
If you want to preview your photos in Windows Explorer you need to download the appropriate RAW codec from your camera's website.
For a Nikon this is the link to the NEF codec download:
http://www.nikonimglib.com/nefcodec/