This is my guide on buying items in all categories of eBay, especially expensive items, such as laptops, desktops, computers, etc.
First thing is first, check the sellers feedback! Don't risk thousands of dollars buying an item off an eBay seller with a low feedback rating. If a seller is genuine, they will have a high feedback rating, as in, over 98%.
Second thing, make sure the seller is genuine! In a listing, if there is something like: "email me at (an email address) as my contact seller link is broken" or something along those lines, DONT trust them! They are saying this as they have most likely hacked into somebody else's eBay account and do not have access to the REAL sellers email address, hence, they make you email them at their own email address. You can test a seller to see if they are real by asking them, a question about the item via the contact seller link. Ask them a question about the item, such as "how much would postage be to my area" or "what bank are you with for bank deposit", just to see if they respond to you. Remember, it is not hard to change your email that is on file with eBay, so if they have changed emails, they can easily change it with eBay.
Another way to test if they are genuine or not is to go through their last sold items (on their feedback page). If they have sold the item you are trying to buy before, and recieved positive feedback for it, they are pretty much genuine. For example, if you are buying a dell laptop, check their feedback to see if they have sold a dell laptop before, and if they have, AND recieved positive feedback for it, then they are pretty much genuine (especially if they have sold more than one of the item). If all they have sold is collectable items (such as baseball cards, etc) and they are suddenly selling a laptop, then they are fake.
Also, make sure their picture is one they have actually taken, not just one off an internet search. If they simple get an image of it off a Google search, then they may not necessarily have the item. Another good idea would be to email them asking for a picture of the item or the box, with something in the photo to prove that they have taken it (for example, a piece of paper with their eBay id)
Thirdly, if pickup is avaliable, with cash on pickup/delivery, then they have to be genuine. They cannot possible sell an item and offer local pickup if they do not actually have the item. Also, make sure they have Paypal's buyer protection program, so if they are fake, you will be able to claim up to $1,500 back.
Finally, NEVER deal with customers outside of eBay. By doing this, you are basically giving your money away. If you make a transaction outside of eBay, then the seller can wipe all of your money out of your pocket, with you not getting any item nor a cent back.
I hope this guide has helped. It is a very bad sight seeing someone losing thousands of dollars because a seller was a fake.
Thanks for reading!