It should be possible to resell Spore and install it on another computer, however, there are a few caveats.
EA dislikes resale of games because it hurts their revenue, and they try to discourage it as best they can.
For example, as Spore can only be installed 5 times in total, every time you installed it will be subtracted from that figure. The new owner will be able to install it only 4 times or less.
Additionally, other restrictions may be placed like no technical support, limited online capabilities and so on.
For more information, see this article on Ars Technica: EA skirts first-sale doctrine with limits on resale of Spore.