Starting last season (?, as I recall), the passes switched to digital. The Purdue ticketing accounts easily link to StubHub so that's where you tend to see the most resale volume.
FYI lots D, B, & T are all on the Ackerman golf course. The D lot is one of the more popular. T is "better" (it is slightly closer and requires a much higher $ donation), but tends to be a little less expensive on resale sites because the masses are out hunting for D passes to meet up with friends and don't realize T is practically the same.
R lot is paved, which is great if it's raining and muddy but suboptimal in the heat.
S lot (Slayter Hill) is pretty popular for tailgating, too - but the topography is highly variable. You might be on a 45-degree incline or you might be in a paved lot on top of the hill.
You can still buy an IM pass from Purdue Athletics now for $30 - - it's not a whole lot further. Those are flat grass fields.