The final value fee of an Ebay auction costing $1,000 is $100.
I don't understand the value of him offering you the same item on Paypal for $99 less. If he just sold it through Ebay, you would pay the $99 extra and that would probably cover the fees. I don't know for sure since you blurred out the cost on the invoice.
Saving $99 is small potatoes compared to what you are paying for the item. You could lose out big time if he doesn't come through on the deal.
How long has he been a member of Ebay? You can get 22 feedbacks pretty quickly. Or it can take a while. I have been on for a long time and only have 200, primarily as a buyer. Then you see these Powersellers with over 100,000 feedbacks, wow. Are his 22 feedbacks from selling? or buying?
I would also look at what other items he has sold. I know that after a certain time Ebay doesn't allow you to look at item listings, but if he has sold anything recently you can look. You can also look at what he is currently selling. You can also see the dates that he has sold things - are they all recent or spread out over time?
I noticed you are in Malaysia. Where is the seller located? It may be more difficult to get your money back if it is an international transaction.
The seller may also not accept a return if it doesn't fit in your machine. And I don't think you would have any way to force him to return your money. You bought the item as advertised. It would be your fault that you didn't make sure that it fit before you bought it.
I personally wouldn't do this deal. I would pay the extra money and get one in your country, either at a store or a legitimate online seller. At least you will have some recourse if it doesn't fit and you need to return it.
Other World Computing is known for their extensive testing to ensure that the item you are purchasing will work in the machine you have. They have a 960gb for $879 USD. You can always contact them to verify if the item will work in your system.
http://eshop.macsales.com/shop/SSD/PCIe/OWC/Mercury_Accelsior/RAID
Personally, I would be tempted to report him to Ebay for violating their rules.
You may also want to try running a Google search on his Ebay user name and his Paypal email address to see if you can find anything.