If approached by a shop assistant in this kind of a situation, I would say "Я поки що тільки дивлюся" (I'm only looking so far) or "Я ще не вибрала" (I haven't chosen [anything] yet) or a couple of other ways to convey the message :)
Note: Я просто дивлюся has a much wider spectrum of meanings than just telling shop assistants off.
Я просто дивлюся. It seems to me i just have seen дивлюсь or something like this. I mean we have ending ся, сь. In one of the lessons i made earlier i saw ся ending for дивлюся. Are these two acceptable forms of the same word in first person - "I watch/I am watching" ? (I haven't read recent introductions to lessons, maybe i missed something)
‘сь’ can be used only after a vowel or sometimes also after the final ‘в’ which sounds like a semi-vowel ‘w’. It is a colloquial shortened variant of ‘ся’ and the latter is always preferable unless you know why you want to say ‘сь’ and not ‘ся’. You often meet ‘сь’ in songs or in Imperative forms like ‘Дивись!’
As for your question, both ‘дивлюся’ and ‘дивлюсь’ mean the same, but ‘дивлюсь’ is more ‘lazy’. And of course if a word after ‘ся/сь’ starts with a consonant it's better to use ‘ся’.
BUT! You always say ‘сь’ in transgressives: ‘дивлячись, подивившись’.
In our western dialects ‘ся’ stands aside from its verb like in most other Slavic languages. Needless to say that in that case it behaves differently.