I had what sounds like a similar problem with a preamp I bought used. My technician found that the pcb for the remote had come loose during shipping and just needed to be put back in place. Cost $100 and seller covered it.
Who is Liable with Defective Unit
Purchased a used (10 yrs. old) DAC on internet...was represented "as new", in perfect working order. Arrived OK, packed well, etc. Plugged in and unit would not respond to remote input. Works with manual input. Remote is OK...can see IR pulses with my phone camera. Sent to factory repair for analysis...PCB problem. Seller will only agree to pay 50% of purchase price if returned, which is also about 50% of the repair cost...claims unit was just fine when he sent it. Pay Pal transaction. What remedies do I realistically have?