I personally would avoid painting and strip down any metla mini's for resale. Whilst there is a market for painted mini's, paint schemes are very subjective and even if you paint a popular scheme like blood angels for example. You re always going to have ultrmarine players who would have bought the models but are now put off. Black undercoat is hard to photograph and so again won't give good sales results. I'd assemble the models and maybe even convert them before breaking them up into units (or independent miniatures in the case of metals) as whilst some people will buy an entire army. The market is more suited for either very small/cheap armies at the moment or individual components.
i always sell on ebay, purely for the security side.