I want to buy a 2011 Mac Book pro but don't want to run 10.7 or 10.8. Can you install 10.6 on them and will they run correctly?
No, and no.
Yes, you can. The key is that you have to purchase the 10.6.8 retail version of Snow Leopard on DVD. Or purchase a 10.6.6 version, install it and download and install the 10.6.8 Combo update from Apple. Snow Leopard 10.6.8 has about 50% of the programming necessary to run Lion and therefore is compatible to run on any Mac that came preinstalled with Lion. Can't speak for Mountain Lion, but I know this to be the case for Lion.
Ran into a similar situation running a graphics shop staffed with Macs and limited funds. They could only buy 2 Macs per year. Got 2 in 2011 preloaded with Snow Leopard and got 2 this year preloaded with Lion. Had to downgrade the OS so that everyone could install the same software.
You might want to take a look at these forum posts for more clarity. Apple says you shouldn't, doesn't mean it can't be done.
I would however suggest NOT trying the above on anything with a retina display. The programming for those was not included in Snow Leopard and a downgrade might be disastrous.
If looking for an older version that's like new, it might be recommended to look at the "refurbished Mac" section of the Apple online store. Can't go wrong...they're refurbed at Apple.
This just seems like a monumentally bad idea based on incorrect assumptions for the sole purpose of reducing functionality and lowering compatibility.