Most used shops have these weird restrictions on what you can do with trade (weird to me anyway, as our trade is good on whatever we stock), presumably motivated by the fear that people will bring in a bunch of icky, boring books and turn them into an awesome, collectible book. Our late, unlamented competitor was a particularly egregious abuser, disallowing whole sections of the store and formats of books from their trade system. If you brought in, say, pocket books your trade slip was only good on other pocket books.
If you know what you're doing, if you buy books mindfully and carefully, you shouldn't care what people use their trade on.
Here's an example of someone bringing in a bunch of okay stuff (solid books, or good books with problems) for trade and taking out a "good" book, defined as a very saleable title you don't see very often. The customer in question is one of the scanner trolls who let their phone do their thinking, but a bit more ambitious than most- he also picks up books he thinks he can trade to us. So, a predatory character who wants to turn common stuff he can't sell online into less common stuff he thinks he can.
Today's interaction had him bringing in about a grocery bag of books which the store could really use, salable authors we can never keep in stock, some neat editions of stuff (a Joseph Campbell book with a DJ, a 1st edition of a later Maugham book that wasn't in great shape) and some cool if not very salable items (a history of a particular theater in Ireland).
Grand total, $25 in trade.
He immediately turned it around for a clean copy of Star Strangled Banger, a Ralph Steadman art book.
By itself, a way cooler book than anything we got. But we're a bookstore, not a library- we want to sell stuff, not stockpile it. I mean you obviously want nice books, but in the context of selling them to a happy customer.
From my perspective, this was a great trade- we were happy, he was happy, everybody won.
Why was I happy when he walked away with the "best" book in the deal?
Because the books we got priced out around $100. Of that, roughly $40 comes from books I know will sell fast. So we come out ahead, even if I totally tanked on the rest of the buy and those books just mope around the shelves for a few years.
From a personal perspective, sure I'd rather sell 'fun' books than boring ones- or buy them, for that matter.
But $40 worth of books is $40 worth of books, whether it's one cool 1st edition or a grocery bag of Agatha Christie paperbacks. The business doesn't care what you sell as long as you make your nut.
Also, this is an edge case- 95% of customers pretty much buy what they bring in. Getting all control-freaky about trade is mostly pointless even if you aren't a strong buyer.