I recently ordered a plant that I discovered was highly invasive in Canada, and I live in the United States (just south of the border), and didn't know about the problem until after ordering.
Are there laws that protect the buyer from their own negligence with plants if such a problem is known before the trade/sale?
I will draw on Washington law (which is, coincidentally, next to a portion of Canada). There are state law regulating noxious weeds, starting with and centered around RCW 17.10. There is a list of weeds, statutorily codified in WAC 16-750 (which also states penalties for violation), and a quarantine list in chapter 16-752. Tying this all together are the county weed control boards, relevant because what constitutes a weed in Kittitas County isn't the same as what counts as a weed in Grays County. The boards establish various levels of sanction against plants that they determine to be noxious weeds. There are three classes of weeds, the "worst" being Class A. An example of a class A weed is Heracleum mantegazzianum a.k.a. giant hogweed. It is
prohibited to transport, buy, sell, offer for sale, or to distribute plants or plant parts of the regulated species listed in WAC 16-752-610 into or within the state of Washington or to sell, offer for sale, or distribute seed packets of the seed, flower seed blends, or wildflower mixes of these regulated species into or within the state of Washington.
A landowner is obligated to eradicate any class A weeds on their land, and must take less-drastic steps against lesser weeds. The county board is authorized to "negotiate" to spread the burden on landowners, per RCW 17.10.154, over some period of years.
These laws impose strict liability, that is, there is no requirement that the person deliberately or knowingly introduced the weed. Rather, the simple fact of having such a weed on your property is against the law. The "knowledge" requirement is that once you know there is such a weed, you must act as specified by the law, or face a monetary penalty.
Plant and seed sellers generally are aware of these regulations and know that they can't send scotch broom plants to Washington, because it is illegal for them to do so. The law does not treat buyers and sellers differently in this respect -- the seller is obligated to know that it is illegal to sell, and the buyer is obligated to know that it is illegal to buy. As such, a sales contract where you buy scotch broom plants in Washington is unenforceable as a violation of law. On such grounds, a sale of forbidden plants could be cancelled. So you would need to look on your state's list of noxious weeds, see what you are required to do and whether you have violated the law in buying the plant -- it is probable that the seller has violated the law by selling it to you, unless you bought it elsewhere and transported it into a forbidden zone. Note that not all "invasive" plants are legally noxious weeds, for example Prunus laurocerasus (English laurel) is still legal to sell, and impossible to kill.
However, if the plant in question is legal and unregulated in your jurisdiction, then there is no legal recourse: the plant isn't defenctive, you simply were unaware that you didn't really want that plant.
