To be especially clear, you won't be able to buy mastery signs for items you haven't actually mastered, but if you've mastered a particular crop, tree, animal, crafting recipe or even flower bloom and want to buy either duplicate mastery signs or replace the one(s) you've lost, you can now purchase a single mastery sign for 500 coins.
If anything, this will allow farmers the chance to have "themed" mastery signs on the appropriate farms as decorations, while also being able to show them off back home (if "home" is still your primary farm, that is). For example, someone that mastered the Candy Cane crop might wish to show off that mastery sign in Winter Wonderland. Buying these mastery signs is completely voluntary, of course, but it's a nice gesture by Zynga to farmers that may have a love-hate relationship with the signs. Now, you can feel free to delete your signs to save space, but not worry about regretting that decision later.
What do you think of this ability to buy back mastery signs with coins? Do you delete any mastery signs you earn, or do you display them proudly on your farms? Sound off in the comments!