For Sale: Carbon Road Bike 58cm

some more Qs if you don't mind!

How long since last serviced?
What's next to need replacing do you think?
Have you looked at the shifter, have you taken it to a bike shop? Do you think it just needs replacing? What exactly do you =mean by "skips" - can you still use all the gears, but just to change from 7 to 8 you have to push the lever through twice? Is it a problem in both directions?

Thank you!

It's been about 4 months since serviced.
Everything on the bike functions and is in pretty good condition, except the shifter.
I've looked inside and one of the teeth is worn down. You can still use all the gears, but have to push through all the way-so two clicks to the gear above and then back down. I know it sounds a massive hassle, but in all honesty I've ridden it like this and it's not that bad.
It shifts the down fine.

To be honest, I think 600 is pretty fair considering some of the parts that are on it.