Mac OS S Snow Leopard Single User - can it be used on more than one machine?
I am not a lawyer, so take this all with appropriate caution.

The Snow Leopard License states


  Transfer. You may not rent, lease, lend, sell, redistribute or sublicense the Apple Software. Subject to the restrictions set forth below, you may, however, make a one-time permanent transfer of all of your license rights to the Apple Software (in its original form as provided by Apple) to another party, provided that: (a) the transfer must include all of the Apple Software, including all its component parts (excluding Apple Boot ROM code and firmware), original media, printed materials and this License; (b) you do not retain any copies of the Apple Software, full or partial, including copies stored on a computer or other storage device; and (c) the party receiving the Apple Software reads and agrees to accept the terms and conditions of this License. You may not rent, lease, lend, redistribute, sublicense or transfer any Apple Software that has been modified or replaced under Section 2H above. All components of the Apple Software are provided as part of a bundle and may not be separated from the bundle and distributed as standalone applications. Apple Software provided with a particular Apple-branded hardware product may not run on other models of Apple-branded hardware.


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


  you are granted a limited, non-transferable, non-exclusive license to download, install, use and run for personal, non-commercial use, one (1) copy of the Apple Software directly on each Apple-branded computer running Mac OS X Snow Leopard or Mac OS X Snow Leopard Server (“Mac Computer”) that you own or control;


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:


  A. If you obtained the Apple Software preinstalled on Apple-branded hardware, you may make a one-time permanent transfer of all of your license rights to the Apple Software (in its original form as provided by Apple) to another party, provided that: (i) the Apple Software is transferred together with your Apple-branded hardware; (ii) the transfer must include all of the Apple Software, including all its component parts, printed materials and this License; (iii) you do not retain any copies of the Apple Software, full or partial, including copies stored on a computer or other storage device; and (iv) the party receiving the Apple Software reads and agrees to accept the terms and conditions of this License. For purposes of this License, if Apple provides an update (e.g., version 10.7 to 10.7.1) to the Apple Software, the update is considered part of the Apple Software and may not be transferred separately from the pre-update version of the Apple Software.


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


  B. If you obtained your license to the Apple Software from the Mac App Store, it is not transferable. If you sell your Apple-branded hardware to a third party, you must remove the Apple Software from the Apple- branded hardware before doing so, and you may restore your system to the version of the Apple operating system software that originally came with your Apple hardware (the “Original Apple OS”) and permanently transfer the Original Apple OS together with your Apple hardware, provided that: (i) the transfer must include all of the Original Apple OS, including all its component parts, printed materials and its license; (ii) you do not retain any copies of the Original Apple OS, full or partial, including copies stored on a computer or other storage device; and (iii) the party receiving the Original Apple OS reads and agrees to accept the terms and conditions of the Original Apple OS license.


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.