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Let the popularity of a toaster (or the number of customers who buy the appliance) equal $p$, and let the cost of the toaster equal $c$. According to Daniel's theory, $p$ and $c$ are inversely proportional. Thus, $(p)(c)=k$ for some constant value $k$. If $p=12$ when $c=500$, then $k=(12)(500)=6000$. So when $c=750$, $$\begin{array}{rl}(p)(c)& =k\\ \Rightarrow (p)(750)& =6000\\ \Rightarrow p& =\frac{6000}{750}\\ & =\boxed{8}.\end{array}$$According to Daniel's theory, 8 customers would buy the \750$ toaster.