Sometimes a shop assistant will come up to you and ask you if you need any help, you can say "No thank you, I'm just looking" and most of the time they will leave you alone. If it's not in a shop and someone is trying to sell you something, eg. on the street, you can say "No thank you, I'm not interested.", or simply "No thank you".

An idea would be: I would like to thank you about your time and offering but I will not buy your product this time.

Thanks a lot Alan!

in buying :) We say "interested in XYZ" in English

about buying****