In a previous blog post I commented how you should be really careful, especially nowadays, when buying a graphics card from eBay. There are tons and tons of scammers, and many of them are really professional about their craft (making their pages and products look very professionally made, like they were big famous corporations, with custom-printed boxes for their graphics cards, and even with custom heatsink shrouds and stickers that look legit, and even sometimes going as far as re-flashing the card's bios to make it look to the operating system as the genuine thing, even though the card is in fact two or three generations older than claimed.)
However, eBay scammers do not restrict themselves to just graphics cards. Whatever the computer hardware may be, if it's in any way related to performance or capacity, you should be really careful.
USB sticks is another perfect example: If you see very large-capacity USB sticks being sold extremely cheap, you should be suspicious, no matter how professionally-made the promotional pictures and web pages may be. For example, the USB stick may claim to be 1 terabyte, when in fact it may be something like 256 gigabytes, or even 128 gigabytes. Like with graphics cards, they may go to extraordinary lengths to disguise this, using custom legit-looking packaging with "1TB" printed all over, the "1TB" capacity printed on the stick itself (in a very professional manner), and sometimes even going so far as to reflash the chipset inside to make it report a capacity of 1 terabyte to the computer.
When you plug such a USB stick to your PC, everything will look legit. The system will report 1 TB of free space, you will be able to write and read data as normal... up until you hit that actual capacity, eg. 256 gigabytes, at which point the stick will just start returning error codes for the extra data. You have been scammed.
RAM chips may or may not have as much capacity as advertised, but even if they do, they might be slower than advertised. The same goes for hard disk drives. And the list goes on.
Just be careful.