wexxam I don't actually know whether the items are supposed to show in Build/Buy, but it looks from what I can find online that they're only supposed to show in BuyDebug.
For the issue with the pet store itself, try installing it into a clean user folder. Move or rename the existing Sims 3 folder in Documents > Electronic Arts, open the launcher to spawn a clean folder, install the shop but nothing else, load a new save in Appaloosa Plains, place the shop, and send a sim over to have a look. Let me know what you find.
If this works, but you have enough other Store content that you don't want to have to reinstall everything in the new folder, there's a way to combine this new install with your previous content. But it's a bit tedious, and not completely reliable, so only ask for it if reinstalling everything else would be a burden.