This isn't a problem, BUT if they were to modify other data, especially that in forms, it CAN cause issues. For example, I reported
this bug last year. By simply right-clicking and inspecting the quantity field element on the shop page, a user can change the item name field client side and buy items that don't belong to that shop - or any shop, for that matter - so long as they know the item's name.
Therefore, validation is necessary to confirm that the item does belong in the shop. I supplied a fix while reporting the bug.
You really don't want users buying out of season items or ones that aren't sold in shops. However, the cost of the item is, luckily, cosmetic only. They can attempt to change it, but the framework knows better and will still charge them the proper amount.