A lack of sequence numbers does not always mean a counterfeit, but it is a good sign. However, I just looked up the sequence in particular that this cel is from. It appears, to me, to be an A1End PAN cel. In the sequence the camera pans down from Char to the guy on the ground and in the cel shown in the auction there's no space where he could fit. Generally a pan cel is a very long (twice as long or three times as long or more than usual) cel with space at the top/bottom to show the whole scene.
It's possible that the reason this cel doesn't have a sequence number is because it was used as a pan cel but isn't a pan cel and that they somehow switch it to include the other guy on the ground later. (There is a weird "jiggle" in animation if you watch it closely.)
But my guess is that this is a fan cel and the original was probably a pan cel.
The fact that the jet pack in the original animation is the same color as Char's suit (pink/white) and the cel has a grey-purple color to match the rest of the jet pack just further makes me think it's a repro. (Some color issues like this do exist on cels, but usually with a "correction layer" overtop the "wrong" color. In this case that's not the case because the error is on screen--the jetpack should be grey and isn't. Therefore there is no correction layer.)
I am not 100% saying this is a fake cel. It COULD be real for a variety of reasons, even with the error and lack of pan cel-quality. But there's definitely enough to make me doubt.
You're wise to be cautious. Save your money for one with a douga.
EDIT: I screencapped and over-laid the animation over the cel on sale and Sayla especially looks out of place. I can't get a perfect line up due to the way the cel is skewed, but her legs look especially weird and the brown "tongue" of her boot is much shorter in the cel than it is in the show... and of course missing the rest of her feet. Also note the inside of Char's left hand (between his legs) has pink inside his pinkie finger but not on the cel. I definitely think this is a reproduction... and probably a pretty old one.
Image:
http://i.imgur.com/0aV0qTH.jpg