The given dissociation reactions and their equilibrium constants are: (1) $CH_3COOH \rightleftharpoons CH_3COO^- + H^+$; $K_{a1} = 1.5 \times 10^{-5}$ (2) $HCN \rightleftharpoons CN^- + H^+$; $K_{a2} = 4.5 \times 10^{-10}$

The target reaction is: $CN^- + CH_3COOH \rightleftharpoons HCN + CH_3COO^-$ This reaction can be obtained by subtracting equation (2) from equation (1) (or adding the reverse of equation 2 to equation 1). The equilibrium constant for the target reaction will be: $K_{eq} = \frac{K_{a1}}{K_{a2}} = \frac{1.5 \times 10^{-5}}{4.5 \times 10^{-10}} = \frac{1}{3} \times 10^5 = 3.33 \times 10^4 \approx 3.0 \times 10^4$. Hence, the closest correct option is $3.0 \times 10^4$.