There was a friendly discussion about seasonal prices for my equipment tables in the comments for Innkeeper, and I thought I'd describe my position on it.
Up front, it seems like a good thing to set the availability of items according to the four seasons, but in truth this couldn't work. To begin with, there are parts of the world that don't experience four seasons ... and there's the consideration of crops grown in the southern hemisphere. Truth is, no simple general solution exists. Even where it comes to northern crops, hay does not come off the fields at the same time as oats, or as wheat does. Apples are not picked at the same time as raspberries, the sheep are not slaughtered at the same time as the cattle and so on.
No, the reality is that every single 'natural' item would have to have a seasonal date from which the availability was calculated, individually determined and measured upon the calendar. As the date passed, availability would soar, then taper off through several months, depending on the product, and then disappear altogether (though of course not in every case). Ale would boom in the fall, last throughout the winter and be gone as the days warmed up and what was left went bad (if it wasn't deliberately consumed!)
I see in my head how the algorithm could be dropped into the present system. The size of the calculating file would balloon and it would be a lot of research to work out the important dates for every item (some would have more than one), but yes, without question, I could do it.
I'm not going to do it. Not any time soon, anyway. I'm not convinced there's a benefit to justify the work, though there might be if there was a higher cause to be served ... an MMO based from the system, say. But not enough for just D&D. I can't see that, not yet.
But I appreciate the thought people have put into it.