DIAGNOSIS That's classic "new machine taste" or, in your case, "old-stored-machine taste." The internal plastic water lines and the boiler need a good flushing to clear out any stale water, manufacturing residues, or cleaning solution from the previous owner.
FIX Don't use coffee beans for this. 1) Fill the tank with fresh, cold water. 2) Place a large cup under the coffee spout. 3) Run multiple cycles of just hot water through the machine. Use both the coffee brew function and the hot water dispenser. Do this until you've run at least 2-3 full tanks of water through. 4) Remove and thoroughly clean the brew group, drip tray, and water tank with mild soap and water. Let them air dry.
IF THAT DIDN'T WORK If the bad taste persists, the previous owner might have used a non-food-safe cleaner or descaler, or there's mold/mildew. You'll need to run a proper descaling cycle with a food-grade descaler (like De'Longhi EcoDecalk), followed by several more rinse cycles with fresh water.
HEADS UP The first few cups from any new or long-unused machine will often taste off. This flushing process is essential. Also, if they used vinegar to descale, that taste can linger; a commercial descaler does a better job without the smell.