While I was looking around for a reasonable priced Webassign Access code I found that it was most important to make sure the Prefix for the code was valid for your course/textbook. You can search google for the Webassign site to check valid prefixes "webassign access checker".
The seller I bought from listed the prefix as part of the listing, and I was able to confirm the code would work for my course, even though the title on the Webassign card was for something different.