You can't sell Anomolies like Radiation Samples to vendors. Someone might want to buy them at the Exchange, though. Anomolies are for crafting new things at Memory Alpha. As far as other items, most of the Starbases in the various sectors have vendors (Earth Space Dock, Starbase 39, DS9, etc; and there's vendor ships flying around the sectors that you can also sell too. You can also just dispose of them in your inventory tray; and you'll get 40% of the value back in energy credits.