So I decided to list my 13 Pro Max on Facebook Marketplace and sold it within a day for nearly $600 more than Apple's trade-in value. Not bad! ($1,550 vs $955). Well, to be fair, the trade-in should be $1,079.15 with tax savings, so around $470 more.
Much better than having to haggle with Apple or Paybright about tax issues. 🥳
Though I am still curious if anyone here will get the tax adjusted from sending in their trade-in. My girlfriend also upgraded to the 14 Pro, and we listed her 13 for now to see what happens. But she has a base 13 128GB so it doesn't seem to be going for much more than Apple's trade-in. If we end up sending her's in, I'll report back if her tax gets adjusted.