Jimmy that pic is a small old school powerhead (surface powerhead - it lays over the top of the tank and keeps movement on top of the water etc) and a hydrometer (measures the salt amount, weight, of the water). He must of ran a salt setup in the tank previously if he gave you a hydrometer. Or a brackfish setup maybe.
Both of those are not worth selling. Keep them. You might find that powerhead handy for your fresh build.