They're not, generally.
ProModeler was a brand released by Revell-Monogram post-merger, that consisted mostly of re-released kits produced by other makers, like Hasegawa, sometimes with additional aftermarket bits like PE added. Some kits were reworked Monogram or Revell pre-merger kits; the 1/48 P-40E, for example, was Revell's pre-merger kit, with some retooling, including a couple of Monogram figures added. Some kits were completely new tooling, like the 1/48 Curtiss SB2C Helldiver kit.
The brand included some paints and other tools as well. There were some car kits as well as the airplane kits.
I'm not sure that there is such a big demand for them. They're not bad, generally, but most modelers today would probably pass them up for other makers' kits, if there are others available. Some are unique for the subject and scale, like the Helldiver.
Remember that on a secondary market, the sale price is whatever the seller and buyer can agree on at the time of the sale. I would watch those sales and see if they sell for the seller's asking price, or if he has to re-post it, and that more than once.
