Please choose a seller, and own study is important.
If the listing hasn't got lots of good photos, you don't know what you're getting. It's not worth bidding unless you like gambling. Unfortunately that seems to rule out most of the private sellers and bring back swords, cos their average photographic skills are usually nil. I very rarely buy bring back swords on Ebay as a result - the photographic evidence on which to base a judgement is usually not there.
So that means going for listings with lots of good
photos - including a series of closeups of the blade - showing the whole of the blade in detail. However, even if the listing has got good photos, it is essential to research the blade in question, as well as considering each photo in detail. Is the mei kosher? Is there anything wrong with the blade? Go over the pictures paying attention to detail. Any flaws? Any hagire? How badly out of polish is it? Are you going to have to spend some money on it and if so, is it worth it? Sword and koshirae can be restored to tip-top condition, but is it worth it and can you afford it? Check feedback as well - any problems?
If you still have questions, any good dealer won't mind running off some extra photos if you need them. I've done it myself, as well as having asked for them. Ask questions. I've had long conversations with a potential buyer before now - I don't mind taking the time cos I want someone to be happy with the sword that they buy. Where necessary, I've also asked questions of sellers.
Finally, look at the returns policy. Some seller offer a 7 day inspection period. I do. That eliminates any remaining risk. You get to look at the sword in your hand. Other say 'all sales are final'.
Any market anywhere is going to present risks. If you haven't got the knowledge, if you haven't got good close-up pics of the blade, or you don't pay attention to the details on a series of good photos, sure, it is going to be a gamble. As Nakamura-san says, your own study is important.