くれぐれも浄水器の悪質訪問販売業者や無料点検商法にはご注意ください。

。

Please be very careful of dishonest door-to-door water purifier salespeople and free-inspection scams.

Literal

Thoroughly water-purifier [possessive-の] malicious door-to-door-sales-companies [and-や] free-inspection-scam [to-には] please-be-careful.

くれぐれも (thoroughly/I urge you) is a strong adverb for earnest warnings. 悪質訪問販売業者 is a dense compound: 悪質 (malicious) + 訪問販売 (door-to-door sales) + 業者 (business operator). 無料点検商法 (free inspection scam) is another compound describing a deceptive sales tactic. ご注意ください is the polite honorific request.