Easy way is just as you intend - don't do it
Of course, I know in my case the spirit is willing but the flesh is weak
OTOH, for my chars, it's very convenient, especially as an alchemist. RPG-wise, my chars reach agreements with some specific merchants, particularly in Balmora, to maintain a minimum quantity of an ingredient with the quid pro quo that my char will buy it in bulk at the merchant's offered price less one drake. Or again in an RPG vein of thought, your char and the merchant might agree that your char will initially gather the ingredients and sell them to only that merchant in the town so as to increase the quantity that the merchant will keep on hand as a way to get the ball rolling. I prefer to think of those merchants as engaged in bulk sales to both retail vendors and specialized commercial and hobbyist operations in addition to their retail trade
All in all, it's not really a problem, and it can be worked around fairly simply by not engaging in that behavior. I will say that I do notice it most often not with the merchants who sell ingredients but those merchants selling weapons and armor.