I used to sell a lot of car audio on eBay, all second hand and I found it to be a very specific market.
My experience was that quality brand name equipment always got a decent amount of interest, and quality prices that reflected that level of interest. It all come down to price with second hand, as it does with most things on eBay, but I could still make decent money on no name audio, but only on limited interest, and mostly from buyers who were not car audio fans, more price buyers.
If I was to go back into second hand sales on eBay, car audio would be high on my list of priorities as a product choice.
For the most part, eBay is flooded with low quality imported car audio, all pretty much garbage for the most part. I would suggest that car audio itself isn't a bad field to be in, but stick to quality products to generate the most interest. If you're a car audio fan, then you have the knowledge to position yourself within that market correctly.