I've never bought a cel, even though I've had tons of opportunities being a regular eBay flyer. Mostly because my sister warns of the insane care required to maintain the artifacts.
Cells ARE a pain to maintain which is why I have stayed away from collecting them. On the few occasions that I HAVE thought about buying that $30 cell of a character I love, I quickly reminded myself that:
1) cels were never meant to last forever.
2) temperature and humidity affect cells, they cannot be kept in an unstable environment i.e. if your bedroom is in the attic of your house and you have no air conditioner -- DO NOT BUY CELLS
3) the safest place to keep an animation cell is in a cell book... in other words you put them in a special acid free book made specifically FOR cells, keep it in a cool dry place, and never expose them to air or light or else they may fade, peel or stick to something. In other words, you never get to look at them.
4) if you DO frame them to put them on display you must:
A) have it professionally framed and matted
B) Have it REFRAMED and REMATTED every year or so just to make sure the "acid free" matt does not eat away your cell
C) Make sure you put spacers inside the frame... if you do not, the cell paint might stick and peel.
D) Make sure you take it to someone who has EXPERIENCE framing cells. People have told me horror stories where they tried to get the thing professionally framed at a frame shop, by someone who never framed cells before, and they more or less cut their cell out >_<
After thinking about how much money I would waste on having the thing framed and reframed every year or so, I decided that it wasn't worth it (and I'm sure as hell NOT going to spend more than $30 on a picture I'm never going to SEE!)
But if you like high maintenence collectibles... cells are for you.