"how would the prosepctive buyer know?"
Most "audiophile" pressings issued during the past couple of decades come sealed in loose bags of fairly thick plastic. And of these most have perforations along one side for opening. While it is possible to open and reseal these bags it is very difficult to do without leaving some indication of the reseal.
Most non-audiophile collectable albums come sealed in "shrinkwrap." The soft thin material is wrapped around the cover, the open edges are sealed with a hot wire and the package is run into a moderate-temperature oven for a few seconds, shrinking the material around the cover.
The latter process can be duplicated at home, but only with the proper material and a machine. It can be done, but takes some effort and investment or access to the hardware.
Over time the shrinkwrap tends to change shape, elasticity, and color. Duplicating a 20+ year old wrap is almost impossible with a new wrap. Additionally, the material used from the '60's through the late '70's was different from later material.
Close inspection of a previously-unwrapped and resealed cover itself will usually reveal some evidence that it was opened. Unless the original owner has been extremely careful there will be some less than perfect edges or scuffing or something similar.
So ... it is possible to "reseal" a package. But someone with moderate experience and a detailed inspection can almost always detect a prior opening.
As to value - the only way to ensure "new" vinyl status (distinct from a "perfect pressing") is for the package to have never been opened. Sometimes even one careful play will generate some noise or visible evidence (or cover wear as pointed out above). For many hardcore collectors anything less than "new" is unacceptable. It can also be very rare, which means the value can be quite substantial to someone who wants one.
