In Arkansas it is legal to buy and sale cemetery, However there are laws that protect the Graves themselves from being destroyed. Especially if they are still reconizable as grave and are maintained.
This does not mean that we are not losing old family cemeterys at an alarming rate. Especially in our rural areas which have grown up in Pine Timber. Logger destroy most of the old cemetery out in the woods with the logging operations if somebody does take care of them.
Anyway more to your point, those old graves need to be marked in some manner that reflects the property to be a cemetery and that someone cares. One of the best ways to make a cemetery more noticeable is to mark those unmarked graves that you are talking about being able to find.
The very first thing that comes up in doing this is how much is it going to cost?
Well, it can get expensive, but anything is better than nothing. A Wooden Cross if nothing else. But I have found a little more permanent marker is a 16"x8"x2" concrete block from your local hardware store buried about 6" deep on its end with the remaining 10" sticking up erect, makes a cleap and easy headstone.
Also if you know any information about the person who is buried in the Grave you can have a small brass plate(3"x5") engraved at you local trophy shop and glued onto the Concrete block, using "J.B. Weld" epoxy, makes a headstone for for about $1 per grave if you don't use the Brass plate, to maybe $7 if you do use it.
I have used this technique to mark about 50 graves using the brass plates in several old cemeteries and they have lasted now for over 10 years with no appearent deterioration. I figure that is about $.50 per grave per year so far to mark those graves.
By the way, I have found that I got the best results when I used the type of brass plates that have the 'peal and stick' backing on them that mounting on wooden awards plaques. The glue on the back of them helps bond with the "J.B. Weld" epozy. Also if the concrete block is rough, or better has a pourous surface, the epozy will stick better if it has something to stick to that is not smooth.
In this way it is possible for even a single individual to mark several graves by himself fairly cheaply. And in the case of marking family members in and old cemetery it is a way to at least put the persons name on a brass plaque headstone over the grave, without having to buy a $600.00 plus headstone.