Politicians answer, both are applicable. I sometimes buy complete bikes to obtain the parts I am after and then sell-on the frame or vice versa. I have also built up renovated frames with both used parts for a low cost build as well as new parts in some cases. I think all bikes have a ceiling price , a bit like house prices in a given post-code/zip (above which people will not be willing to go). My rule of thumb is not to get carried away adding niche or expensive parts to a build you intend to sell and don't personalize it too much, as people will not pay over the odds for those tasty parts and they won't share your taste either.
I would also say if the frameset is a common one then low price and popular sizes are most important, but if it's a rarity, it may not sell at all unless you build it up so people can see what the end product is going to turn out like. If you try to sell a tatty old frame that needs a lot of work, you will probably either not sell it at all, or will lose money on it when you drop the price sufficiently to sell it. In general I try to cover my costs and rarely make any profit at all and have never recovered anything like the time and effort that has gone into a build. I don't believe it is possible to generate any meaningful income buying and selling bikes and parts, unless you are acquiring them yourself with very little outlay up front.