At that size, most predatory fish would do the job. So then tank size considerations would be your biggest limiting factor. If you are looking to trade or sell down the road, stick to colorful and or unusual species. The more aggressive they are, the harder it will be to give away since not everyone will readily have a tank that can suit it, such as wolf fish. Pike cichlids come in all sizes and colors and would be a good bet. As long as the fish are in good condition you shouldn't have trouble getting rid of most species, as long as they are not true monsters like rtc. Even Oscars are fairly easy to give away if they are in good condition with nice colors,but if the fish are cheap and common line Oscars, I wouldn't expect to receive much, if anything, for a trade or sale price