There are many reasons why someone might list items for a sale on Ebay. Usually it is because they have something that they want to sell. Most sellers don't have any thing to hide, they just want to sell something. The pitfalls of buying on Ebay are the same as buying used equipment mail order anywhere. You don't know what you are going to get until you get it. I have found that most people describe their merchandise accurately. If they say it is in good condition, it usually is. If they say that it is "as is", it is probably not in very good condition. I have seen a number of strobe heads that have been pretty badly beaten up.
A few rules: Never pay more than 50% of new price. If it sells for more than that, let it go. Don't bid until the last moment. If you bid early, you just drive the price up. If you can't bid at the last moment, bid your maximum price and let it go. Someone else will probably outbid you. Pay attention to the seller's feedback rating. If it is less than 99% positive, you may not want to bid. Remember that you are looking for a bargain. If you are willing to pay new price, buy it new. Check B&H and Adorama for the current new price. Use those prices as your basis, not manufacturer's suggested retail. Pay attention to the sellers terms of payment and shipping costs. Sellers will often tack on high shipping costs that you may not notice, until you are the buyer and you have committed yourself. Pay attention to the seller's location. Don't bid on overseas auctions. Don't send money overseas. Watch for out of place listings. If a Balcar head is listed under Norman, it probably won't bring a good price.
There are some good deals on Ebay. Be aware that popular items are going to command premium prices. Add some items to your "My Ebay" watch list and see what they sell for. In that way, you get a better idea of what other people are willing to pay.
I hope that this helps.