I'd collect it in a plastic bag and return to him, letting him know he forgot something :)

My husband says he would go and squat in the neighbours yard

I would take a picture of the action,  post it on the local social media page with the following headline:

" the next door neighbor and his dog have already decided to make himself at home."

I put together a realtor supply kit that I keep in my trunk.  It includes doggie poop bags.  I'd clean up the poop and toss it in the trash.  

The next time I saw the neighbor with the dog, I'd mention that they must have been out of poop bags when they allowed their dog to poop on my client's lawn and then leave wihtout cleaning up.

I'd say that I always carry extra poop bags and that I'm happy to provide to them when they forget to bring some with them. 

Paul,

I guess this is another contingency that an agent has to be prepared for by carrying dog poop bags as part of the car kit.  How sad is that, when people do that, to use dog poop to express their live's frustrations.  I do like Susan Emo answer, it made me laugh.  A

Got a shovel in the trunk?  they work great for throwing this stuff.

If I saw them leaving without doing a pick up I would call them out.

Just ignore them, not worth your time to participate in the neighborhood drama.

Just have to clean it up and move on.  What if potential client shows up at that exact moment that you are confronting that person

Wow, that is pretty rude...

YUCK!  and YIKES!  With the foot traffic expected, and to avoid a possible altercation, I would bite the bullet and pick it up.  

I am stunned that someone would do that. Were you watching and did he know you saw it?

That is just plain rude. 

I would ask the person if he/she needed a bag to clean up the mess. If that person ignored me and walked on, I would clean it up myself. 

If I see the person in action, I would step out and ask him to clean up after the dog.  There is nothing wrong with that. 

.....

"..and I quit show biz for this!!!" would be the thought running through my mind as I cleaned it up.

I would however weave this into a conversation with the owner suggesting an 'above the call of duty' commitment must be reflected in their letter of recommendation and review.

On my own yard I posted a warning of "Lawn treatment that could cause hair lose, diareaha, restless leg, vomiting, hairballs, scalp itch, spider skin and yellow eyes. Could cause discomfort for pets also."

I have given a free pass (?) to my neighbors dogs because I have a large inviting lawn in a neighborhood of no lawns. In the end (?), no one gets hurt

Make him buy the house? "You soil, you buy!" Not much one can do when others act inappropriately. The neighbor should've cleaned it up, even on the street and not brought the dog over to the Open House in the first place.

Paul - Well, if I would have seen the neighbor and his pet doing their thing, I would have stepped out and told the neighbor that they had forgot something. If they didn't pick it up, I would have to clean it myself, but I might have to return it to the rightful owner if I knew where they lived. I would also let my clients know what happened.

There is nothing you can do unless you wish to confront the neighbor. Just tell your client that he or she has some cleaning up to do.

Bag it and put in trash. Get a bucket of water rinse the sign, then mix 1/4 cup white vinegar with 1 cup water and splash on the sign (eliminates odor of urine that dogs find as attractive) next - dry chili flakes in the grass poop area. Owner can tell neighbor they had grass treated and it may be sensitive to dogs, so walk on sidewalk...  May sound passive agressive but confrontation with neighbor is not going to work, they allowed dog to do this to spite you.  Chili flakes are not harmful, they make dogs sneeze, the vinegar fades. Your goal is to keep front poop free so buyers don't walk in it. 

Paul, very good question.

Having a dog, I would just pick it up and put it in the trash. I know there are a lot of pet owners who do clean up after their pet, but there are always some people who think they are not responsible for picking up after their pet. 

The dog would not have gotten an opportunity to assume the position had I been witnessing the action. It's tough to show respect to those that have none.

Clean it up if it's poop.

I am glad it has not happened to me.

I would report incident to owners and let them handle it. If that didn't work, i would ask seller for permission to handle it myself, and that might invole having police serve neighbor with trespass notice. Would also check on HOA rules.

Susan Emo had the best answer 

I probably would pick up the "business" myself if I had a plastic bag, rather than offending a neighbor.  I have to say that is pretty rude of the neighbor though.

Ugh! Pick it up and smile while telling the neighbor to have a great day! (They'll feel guilty.) Irresposible pet owners drive me crazy, but when you're representing your business, and it's a business where word of mouth is SO important, it is best to keep that smile on your face when dealing with the public. (Unfortunately.)

I'd just clean it up for the neighbor.

......then promptly relocate the poo to the proper owners front porch, letting them know they forgot something.  Irresponsible pet owners are a big pet peeve of mine.

Wow....I think I would call the owner of the house and have him or her speak to the neighbor. Since it's not my property, I'd rather not say anything. As for whether I would clean it up....that would really test me!