Simple, dont buy wet weed.

1] If its done by weight, your being ripped off so much its funny.
2] Never seal wet weed. Wet & Dark = Mold > DEATH. [Or extreme pain]

If you are ever unlucky enough to get ripped off with wet buds, hang em up untill the stem snaps easily, and then you can smoke it.

Also, wet weed isnt as potent as cured buds, and the curing process activates THC, and breaks down chemicals such as chlorophyl, so you dont get a plantly smell/taste.