The nature of plaster is that it needs to remain submersed. Once it is exposed to the air (and sun/high temps) it starts to "release" from itself. That is the main reason that pool owners in AZ, for example, do not drain their water during a good portion of the year, as it is so hot that the plaster fails quickly.
PebbleTec (and I would assume, the knock off products to it) hold up better to this type of exposure, as it is comprised of mostly rock, and has only a little plaster to hold it all together. P-Tec is the only product in the line that is capable of continuous exposure (P-Sheen and P-Fina are not).
You may need to educate the Realtor as to what happens to plaster that is no longer submersed, and the costs associated with re-plastering a pool. They may not know what the effect of this is, and are only trying to keep from having green pools in the homes! In the end though, they are really hurting the new buyer, who probably also does not know what he is in for from the pool being drained and exposed.
Bruce