So what happens when Ladder resets

Excluding trading for a second, a ladder reset means absolutely nothing for you if you're currently on non-ladder. If you're on ladder, it means the tag of your characters changes from ladder to non-ladder which kind of also means absolutely nothing. The only impact for ladder players there is that you need to move your shared stash into your NL one before the next season ends so you don't lose that stuff.

At the same time, going into ladder post-reset means you're looking at even footing as everyone starts in a fresh, new season and noone has anything yet.

Now, in terms of trading, the impact for ladder folks is quite significant as the trading economy early season tends to be vastly different from mid and late season. People tend to "overpay" hard for any of the more core pieces early on, even the "easy" ones like

Shako and whatnot. Great time to farm and immediately sell (and then buyback what you yourself needed later at a massive comparative discount).

For non-ladder, most trading doesn't change too much anymore. There does tend to be an influx of new items as everything from ladder is converted down but since we're multiple seasons in, the impact shouldn't be all that significant anymore. NL is fairly matured as is. The obvious exception is ladder-only gear which will also trickle down into non-ladder, can now be sold and will likely be far cheaper on and shortly after "season day" than later on (educated guess on that one mind you, didn't actually track that) as most will immediately try to sell their ladder-only goodies, drive each other down hard, only to have prices pick back up as most are sold and only a patient handful are left once we're back to mid-/late season.

Beyond all that, season end does tend to come with a lot of giveaways from ladder-only folks who don't need their converted stuff anymore so if you're into getting free stuff (and often top-end at that), keep an eye out for 'free stuff' games. Though those often already happen the day of reset (before the actual reset) on ladder itself so they have the char slot(s) cleared and ready for the next season. I.e. make a ladder char on the day/day before and have a look.