It is unlawful to "discriminate in any aspect of selling or renting housing or to deny a dwelling to a buyer or renter because of the disability of that individual, an individual associated with the buyer or renter, or an individual who intends to live in the residence."

If the property is in SoCal, the process is as follows:Serve a 3-day notice, and assuming they do not pay, you file an unlawful detainer. 

Can anyone think of reasons that this is a bad idea or may be unlawful in some jurisdictions?

Since these folks are not tenants, but the seller who has not vacated the premises, you really have an unlawful detainer on you hands and an eviction may not be the appropriate remedy.  

And I'm not a lawyer and don't live in CA but:http://law.onecle.com/california/penal/25605.htmlhttp://law.onecle.com/california/penal/26035.html"FIrearmS In tHe Home, buSIneSS or at tHe CamPSIte Unless otherwise unlawful, any person over the age of 18 who is not prohibited from possessing firearms may have a loaded or unloaded firearm at his or her place of residence, temporary residence, campsite or on private property owned or lawfully possessed by the person .

Our local rental association keeps a list of Unlawful Detainers that were filed, but it is printed on a month-to-month basis.

If they can move quickly and avoid eviction/unlawful detainer on their legal record, it will be better for both of you. 

The only real issue here is a tenant who wrote you a bad check, and is trying to get out of paying rent, even though you keep bending over backwards to fix everything.Time to grow some cojones and start eviction and let her hire an attorney.And the beautiful thing about that is - that whenever an unlawful detainer is filed - it goes on her eviction record.  

If they have unlawful detainers / evictions on their records, I won't rent to them period.  

Furthermore, going on to your property without permission is unlawful and should be handled appropriately.