I've been noticing in a lot of pictures from other players that you all put the cards in sleeves. is that purely to protect the cards or are they collectible? I've also notice some cards being sold on ebay like they are rare, even though each expansion has all the same cards. Am I missing something, is there a collectible aspect to this game I am missing?

Crunchbacca

I and many others put our cards in sleeves just to protect them. There's nothing collectable about most X-Wing cards. But they may be hard to replace if something gets spilled on one.

I for one would rather not buy another YT-1300 just so I can replace copy of Han Solo.

There are some cards that are collectable though. At most official tournaments, and in the game night kits they include alt art copies of existing cards.

For example at this year's regional I got 2 copies of an alt art version of S&V Boba Fett. Since the only way you can get those is at the regional, some people who couldn't make it may be willing to pay for them.

+1 to the above. Also, I own four lambda shuttles, who needs four Col. Jendon cards? I only need two at most, one to use and one backup. The same goes for most people who bought multiple of the same expansion, they might want to cash in extra pilot cards.