I'm afraid its a question no one can answer. We don't know anything about the item or the person whos selling it so I can't see how anyone can comment. You haven't even provided a link to the item in question so people can even look at the feedback.
Most people on Ebay are honest and just trying to make a few quid, some of course are con merchants but they are the minority. Having said that though I'm not keen on buying second hand memory, especially with the compatability issues. Thats why I always use Crucial. You say its the correct memory and I presume you are certain this is the case and that you have checked your laptop manufacturers website to check that it can take the extra memory.
You pay your money and take your chances I'm afraid.