I think it's ok as long as you own the cel as
well.  If I use other's scans I always try to make
sure and give them credit for it, I know how time
consuming it is to scan the cels and they deserve
any credit.

If you still have second thoughts about it you can
always ask the seller, but in your case I'm sure
it's too late for that since you might have had
them for a while.

Maybe you could post the sellers scans for now and
then after time scan them up yourself and
eventually replace them, that way you can show all
your cels sooner.