Dishonest door-to-door sales are occurring frequently, such as conducting unauthorized water tests and then pushing water purifiers on people.

Literal

Without-permission water-quality-test [object-を] conducted [after-all-that-あげく] water-purifier [object-を] force-sell [etc.-など], malicious door-to-door-sales [subject-が] frequently-occurring.

あげく (after doing all that) carries a negative nuance — the testing was a pretext leading to the unwanted sale. 売りつける (force-sell/push on someone) is stronger than 売る. 多発している (frequently occurring) uses the formal compound 多発 + ongoing state.