So my theory with the shops is rather than adding custom recipes, to allow players to sell multiple stacks for 1 diamond. 
Basic idea would be that different type wood signs allow a different amount of items to be sold. So that would be be to continue using Oak for 1 instance of the item to be sold, Birch two instances, Spruce 3, Dark Oak 4 and Jungle 5. 
This is probably relatively difficult to code but I think it is in line with wilder style and is similar to how we see they do trading on the Hermitcraft server. 
Alternatively, a 4th line of code in the shop signs where it is 
$
Item
Quantity
Price

Quantity being the new addition, so you define it for say you sell elytra quantity 1 for just one set of wings, or selling sand then quantity 1`28 for 2 stacks for example.

I am not sure which of these two ideas is more easier but I hope they are received well.

My main concern is that it would not be intuitive for players who aren't familiar with the system, which can cause some issues that a Mod will likely have to resolve. 

im personally more fond of the Diamond Nugget/Shard options for this reason. Partial Diamonds feels like it would be much more understandable to a new player. 

however, I appreciate the concept in general. Being able to sell multiple stacks for an item would be nice. Signs is an interesting way to achieve this, but it DEFINITELY needs some visual component to help with comprehension. Perhaps when you open the chest, it also displays text on your screen saying (X Stacks per Purchase) or something.