I picked up my little amp from a truck stop CB shop.
When doing business on Ebay, I tend to look for vendors that have been there for several years, served hundreds (if not thousands) of customers, and have a rating in the mid 90's or higher. I also read their reviews, and look for themes. For example, I may note that most of the commentary may say, "Product shipped quickly, and worked as it should."
I have read reviews on a vendor, in the past, that indicated that the vendor had a written "no return" policy, but made an exception for a particular individual. My guess is that some guys will have an official policy of "no return", because they've had problems with people hooking up some things wrong, frying it, then trying to return it, claiming "DOA". The "No Return" policy allows them to "feel out" issues, case by case, and gives them an out of they think they are dealing with an idiot.
You may wish to contact the vendor that you want to use, to ask him, specifically, if he's willing to take a return, if the product is "DOA". If he is a highly rated, long-time vendor, he probably won't have a problem with it. He probably tests each unit before shipping, anyway.
Although, I found a guy, on Ebay, that rebuilds Northstar engines; he was highly rated, had good reviews, been on Ebay a while, many happy customers. So, I shipped my Cadillac out to him, had him rebuild my engine. Without going into detail, I am very unhappy with his work.
It's a crap shoot, on the web. The best you can do is try to find something that shifts the odds to your favor. But, good odds don't guarantee a win, every time.