Maybe it's easy to be a different critter and impossible to be a not-critter at all. Then it only has to have happened once, critterizing.

That would sort of match with how the children of a critter and a noncritter are always critters.

I'm not sure if that'd be the same mechanism but it's at least not in obvious tension.

Yeah, it could easily be two unrelated things.

She sends, and goes to bed before the reply arrives. In the morning, she sets off for the Avalon magic shop, her backpack containing several copies of the reverse-engineering spell and a few of the glowing spell for demonstration purposes.

The shop is there as ever, nestled between a bar and a bookshop.

"Hello!" She says to the shopkeeper. "I'm interested in renting some of your items for a short time rather than buying them outright; is that something you're willing to discuss?"

"...Not something I usually do," he remarks.

"I have," she takes a deep breath, "a spell that can determine what runes went into enchanting an object. I was thinking I could help identify some of the things you aren't sure about. And I'd also like to take a look at a medallion." She twists a strand of hair between her fingers and awaits his response.

He frowns. "I can't really come after you with a lawyer in a court of law about it if you go all copyright violations on me."

"Of course. I wouldn't dream of messing with your intellectual property. I'll only do it to things that you didn't make and don't know how to make--the unidentified stuff and the medallions."

"If you bust a medallion I'm out full price for it even if you didn't mean to."

"I'll pay full price for anything my examination detectably alters."

"You're trying to rip me off, there's not gonna be a way to check if you break half the stuff."

"Forget about the medallions for now, then. Does any of your unidentified stuff discernably do anything now? Or is it all non-magical as far as you know? If you can't get it to do anything now, and you still can't get it to do anything after I've looked at it, you're no worse off. If you have anything that does anything, I can look at that and you'll be able to tell if I broke it."

"You've got a funny idea of being worse off. If I tell somebody, oh, I got this from so and so, they say it does a thing, caveat emptor," he says, "and it doesn't, then they don't have a leg to stand on. If they come in and tell me it doesn't work and I say, oh, right, I let that kid experiment on it, then they do."

"So how much do you want to charge for that risk, versus the gain of ending up with a working thing that you understand? Or if you don't care about finding out what your things do, what's the cheapest kind of medallion you have?"

"My reputation's worth more than whatever some kid tinkering with runes can tell me," he snorts. "You pay full price for anything you want. Cheapest medallion is this tikbalang one, hasn't been a tikbalang in town for thirty years and I dunno how to sell it to somebody in the Philippines."

"How much is it?" She makes a mental note to check how geographically distributed her invisibility jewelry customers are. Presumably they're all from places that speak English, but still.

"You can have it for two and a half grand."

"When you can't sell it and I can't use it? One thousand."

"I'll get eight for it if I ever find somebody to translate the sale post into Tagalog," he snorts. "I'm not a fruit vendor, take it or leave it."

She buys it, because further haggling would require interacting with this guy even more. 

She gets it in a nice little bag and he tells her to have a nice day.

"You too!"

She gets some lunch to go, eats it on the way home, and starts doing some tests. First test: can she get the same diagram out of the same glowing rock a bunch of times, or does it only work once per rock? She can't think of a reason it wouldn't work repeatedly, but it's good to have a baseline before doing more uncertain things.

She can get the same diagram more than once!