That can happen at train shows. Some good vendors will let you test run before purchase. My module group had a test track that people used for testing HO scale items. It was DCC and DC through a DPDT switch.
Sorry but it happens all to frequently.
Pete.
I purchased a Broadway Limited Inc. T4 Pa. Duplex 4-4-4 N scale from this vendor at the Altoona N show in Altoona. It was discounted. The vendor claimed that it was new but I should have suspected that it was not when the packaging was opened. I suspect that it was refurbished. None the less, it does not run. National does not return my phone call or emails. Fortunately, BLI will repair it under warranty. Buyer beware!
DMK