Does this only apply to the family software? I purchased a single user copy of OSX Snow Leopard so that I could upgrade to lion. Once I have upgraded my memory, I will download lion. could I then sell my copy of Snow Leopard? can someone else use it once it is off of my home Mac? what if I sell the disc now? can someone else use it?
Technically, there is nothing preventing you from using a single-user copy on more than one computer at the same time. The system is the same as if you had purchased a family pack. What you are actually paying for is the license. The single-user license states that it can be used on one computer at any given time, while the family license can be used on more than one computer. This means that, once you upgrade to Lion, you can install Snow Leopard on a different computer without breaking the license agreement. The license also allows you to transfer ownership to a different person, which means you can sell the disk and they can install it on their computer. However, if your hard drive fails and you don't have a backup or recovery drive, you will need to purchase either another copy of Snow Leopard or a Lion installer, since you won't be able to download it again.
I am not a lawyer, so take this all with appropriate caution.
The Snow Leopard License states
While you cannot "sell" the software (because you don't own it; Apple does, and licenses it to you), you can make a one-time transfer of your license to someone else (and could charge them for it), so effectively, yes, you sell your copy, as long as you don't keep Snow Leopard around after that.
So what happens if you upgrade to Lion?
The Lion license states
So the computer has to have Snow Leopard installed at the time you install Lion, but the Lion license never states you have to continue to own Snow Leopard. So perhaps if you install Lion and Snow Leopard is obliterated from your computer, you can sell Snow Leopard -- but wait, there's more.
Under Transfer, the license states:
So you can't sell 10.7 because you're now using 10.7.1. But 10.7.1 isn't a separately licensed, separately purchased piece of software. They don't explicitly say anything about a separately licensed version. So no road blocks yet... BUT
If you sell your computer, and it didn't ship with Lion, but rather you upgraded it via the Mac App Store, you are not allowed to transfer your copy of Lion to the new owner. You are told to re-install the operating system that shipped with your computer, which, in this case, should be Leopard or earlier. So you can't downgrade to Snow Leopard, because you've transferred your rights to that operating system to someone else.
All in all, if you want to be sure, seek legal advice from a competent lawyer, but from a layman's reading of this, it sounds legitimate to me to sell SL once you've upgraded to Lion.