As to a used Glock, there is no significant difference in used prices from one generation to another. YOU may pay more for one than another perhaps because you want finger grooves (thereby requiring a Gen 3), or prefer not to have finger groves (therefore wanting a Gen 1 or 2). The key to value is condition, not generation.
It is true that the long slide G34 and G35, or G17L command a higher price used, but that is not due to any generational issues; also, the full size big boys...the G20 and G21 are usually more pricey used (as they were also when new).