I looked at both sites, and I can see that the Lua-logo'd merchandise is:
Difficult to find
Difficult to distinguish from non-programming-related items under "Lua"
Limited in variety
IOW, poorly marketed. Which is not terribly surprising, since techies are, as a group, terrible marketers. Has to do with why we are techies, and not rich (usually).
If anybody wanted to do some very elementary marketing changes, I think you might see more than 1-2 T-shirts sold in a year. Some really simple tweaks would include:
Addition of larger variety of items
Some new images (I thought the sample program was interesting, but there might be some better/funnier alternatives possible)
Something that would make it easier to find and distinguish Lua programming items (like maybe, adding "programming" or "language" to the search terms)
Including a links page on the wiki to specific items with a short description of each
I suspect that the marketing would actually be *more* effective if you went with either Zazzle or Cafe Press, and not both.