I'm looking for a colloquialism for:
shop around a bit before you make your decision
I think I'd just say shop around (never mind the "a bit before you make your decision"). And so would Smokey Robinson's mama.
"Buyer Beware" -Caveat emptor /ˌkævɛɑːt ˈɛmptɔr/ -Latin
The phrase caveat emptor arises from the fact that buyers often have less information about the good or service they are purchasing, while the seller has more information. Defects in the good or service may be hidden from the buyer, and only known to the seller. Thus, the buyer should beware.
Don't get sold a lemon.
"Can I help you?"
"No thanks, I'm just {browsing / window-shopping / comparing prices}".