It should say at the top of the item description page, before the price/bid info...look for "used", pre-owned, "as is", even the word "vintage" can mean new/not new. If not there, then scroll down to check any for additional info from seller. Not all sellers add that info, but you can always message the seller for any missing info. If seller answers that item is new and you buy item based on that info, you can return item if seller didn't tell the truth...within reasonable amount of time.
In the last few years I've only had to return a very few items, always calling ebay customer service first to clear the way. There's also a complaint system in place which customer service can explain if necessary.
The timeliness of return is very important, especially if you're returning anything electronic.