I have unwanted guests in my home that have removed my "for sale" sign and my lockbox so that potential buyers cannot enter the home. Is this legal?

Ordinarily a "guest" is someone who is invited.  Are these people relatives?  Do you reside in the house?  Do they reside in the house with you?  If you do not reside there, did they break into the home?  Are they strangers to you?  You have not given me enough information to respond in any useful way. Certainly, if you own a home and want to sell it, and if someone (even a relative) enters the home against your wishes and tries to thwart your attempts to sell, they are nothing more than trespassers and should be evicted.  Go to see a good real estate attorney in your area.