If a minor's property is being deal with, the guardian has to take court permission
Otherwise the sale is voidable by the minor and not void
There is a difference between void and voidable
In case a minor's property is sold without court permission then the minor can avoid such sale on attaining majority. This is called voidable sale.
However the minor has to avoid the sale within the limitation period which is within 3 years of attaining majority
Guardians do not have any absolute right to sell a minor's property. So if a minor challenges the sale by a guardian on attaining majority within the limitation period, then the buyer cannot say that the minor was represented by a guardian.