Flashing describes taking a non-Verizon or non-Page Plus CDMA phone, and "flashing" Verizon or Page Plus software on it, including web/data/picture messaging settings, so that it might work on Page Plus.
For instance, if US Cellular and Verizon sell both the same model of a phone, you could theoretically take the US Cellular model, flash it, and make it work on Verizon. IF they are the exact same model and Verizon has software for it publicly available.
A Verizon phone does not need to be "flashed" to work on Page Plus.
Be sure the buyer you buy from eBay has PHENOMENAL feedback - I personally do not deal with anyone with lesls than 99% positive feedback myself. And be sure you get their guarantees in writing.