How do I even figure out what a box of old dev boards is worth?

I've done a few of these garage crate hauls and the way I handle it is to test everything first, then sort into three piles: keep, sell, recycle. Skipping the testing step is how you end up keeping dead boards for five years. To check if a board is functional, all you really need is a USB cable and your computer. Plug it in and watch for the power LED, then check if the operating system sees it on the serial port. If the board shows up and takes a blink program without complaints, it works. The older stuff can be a bit trickier since some of it needs a seperate programmer dongle rather than USB, but even just getting a power light and no smoke tells you a lot. For the LCD screens, that's harder to verify without a matching driver board, so I'd price those cheap or just keep them as future project parts. On pricing, individual boards almost always fetch more than a bulk lot, but it takes way more of your time. A crate like yours might only have two or three items worth real money anyway, like a genuine branded dev board or a sensor that's still current. The generic clones and mystery modules sell better as one lot to someone who wants a grab bag. Take photos of everything laid out on a table, list the good stuff singly and lump the rest together.

Age matters less than you'd think as long as it does the job. An old board that blinks an LED is just as useful for learning as a new one. Where age does hurt is when the software support has moved on and you're fighting toolchain issues, so factor that into whether you keep it. My suggestion on the keep versus sell dilemma: keep anything you can name a project for in the next six months, sell the rest. The parts drawer fills up fast enought from future hauls anyway, and cash from the duplicates buys the sorted storage bins that actually make the drawer usable.

One thing I'd add on the mystery modules, don't guess, read them. Almost every board has a part number printed on the biggest chip, and a quick search of that number usually tells you exactly what it is and what it's worth. I found a stash of modules at an estate sale last year that looked like junk until I typed one chip number in and realized they were radio boards worth actual money. Also don't toss the breakout boards with the connectors still attached, people buy those on thier own for prototyping. What microcontrollers are in there btw? Makes a difference on whether they're keepers.

My haul story went sideways until I started labeling as I tested. I'd have sworn I'd remember which of the mystery boards did what by the next weekend, but nope. A cheap label maker I got for organizing cables in the garage ended up being the thing that made the whole sorting session actually stick. Everything that passed the smoke test got a tag with what it is and the date I checked it, and anything untested got taped to a 'check me' pile so it didn't sneak back into the keep box. Went from a junk crate to somethig I actually reach for when a project needs a sensor.