Oh Wow! You Look great in that dress!!

Thats what the sales person at the store has just told you... fooled you into buying that outfit. She just made a sale!
Sales people... regardless of where they are... in what industry.... may be great at their job but not all of them ( none of them) want the best for the client.
So don't get fooled by "that dress looks fantastic on you",  'cause you might just be looking really bad it in.
I've reached a stage where when I try an outfit on, if they say it looks nice... I say, " Yup, I know, I'll you know if I need anything". So I'm not rude, but telling her that I can make do without her advice.

If you know what looks good on you, go by your own judgement, if you don't, take someone who can be of good help.