Do you have any advantage in either the quality, price, or consistency of your water, salt, vinegar or seeds?
In 99.9% of cases the answer is "no" and procuring ready-mixed mustard is more convenient, cheaper, less messy and time-saving.
In 99.9% of cases the answer is "no" and procuring ready-mixed mustard is more convenient, cheaper, less messy and time-saving.
Store bought mustard and mayonnaise contain such surprising ingredients like e.g. sugar or copious amounts of salt.
Also a little bit of sugar in mayonnaise is fine - in the US it usually relatively trace amounts to the most commonly sold ones in the Commonwealth.
You will probably also not be able to make at home anything all that similar to the mayonnaise you buy in a shop, though what you can make at home will almost certainly be equally as nice (my children were confused by the yellowish colour of home-made mayonnaise, which is sort of backwards since the confusing thing ought to be why shop-bought mayonnaise is white!).