Eli,

PMFJI.  I am only a beginner with HTTrack, but I believe you
can only satisfactorily download websites (or portions
thereto) that are wholly self-contained and which do not
have circular hyperlinks.

In the Mushkin site you offer as an example, some pages are
password protected and there is most likely a background
database engine to provide product information and pricing
dynamically.  Search and shopping cart functions are also
not likely to be copyable by HTTrack.

Websites that are static, that is, having all the
information on a page not subject to immediate change and
all pages organized in a strong hierarchial structure are
prime candidates for copying.

What pages in particular are you not being able to copy? 
Please share the URLs.  You may have already copied all that
is copyable from that site.

--
RJ Emery