I'm so sorry you've had to deal with so many crappy people, Michelle.
The sales assistant sounds like a silly cow. From the way she treats customers I'll bet she's gathering a stack of complaints at head office. Eventually it will get to the point where her lies aren't going to cut it any more. Bosses aren't that stupid. Try to put her out of your head. She's digging her own career grave and she's not worth the energy expended being angry at her.
As to the other people, the fact that they have to put you down all the time says a hell of a lot more about them than it does about you. Why do they need to belittle you, whether it's to call you 'fat' (and at 57kg you are sooo not fat!) or 'vain'? Usually it's the only way they can make themselves feel better. They must have a sad empty little existence to need to pull others down as the only way they can feel good about themselves.
I know it's easy to say 'don't think about them' and not so easy to do, but they're really not worth the anxiety. Sad little people doing sad little things because they know they'll never match you!
If you feel you need help with your self-esteem then counselling would be a very positive step to take. And as Fiona suggested, surrounding yourself with nicer people would definitely be a good thing!