I could extract more profit from a piece if I mixed my own glazes, but would lose some plausible deniability, particularly if ever there were a court case.
It is my understanding (and has been for years and since consulting lawyers on this subject as both a pro potter and as a ceramics educator) if you are located in the USA you have absolutely no protection in a legal situation from the use of commercially mixed glazes. The manufacturers do not indemnify the end user of the product. Indemnification means that the manufacturer will defend you from liability in the case of any issues arising from the use of their products in your situation.
If there was a problem with their glaze or clay body or ?????, YOU are still the responsible party if you are named as the defendant with a custoner's issues with your wares. Your only recourse would be to then take the supplier to court yourself in a separate case to recover any damages that you might have incurred. You can bet that they have a bevy of lawyers on retainer that will be preventing this from happening... and you will be paying your own lawyer for every hour expended.
In fact if you look, you will likely find that in the suppliers or manufacturers catalogs, websites, and such there is a carefully worded legal disclaimer that states that they are not responsible for the uses that an end-user puts their materials to. THAT will be their first line of defense agains you taking them to task.
Just so you know.... you can take the same glaze on the same claybody, and by changing the application or the firing, affect the leaching of materials out of it. In one case it may be "good" .. and in the other "bad". The manufacturers cannot be responsible for what the potter does. It is too complex to control. They are not being unreasonable in this statement.
Additionally there is the Law of Merchantability... which basically states that a product sold must be suited for the intent for which it is sold. A dinnerware piece not suited for food is not going to conform to this. You'd not have much solid ground to stand upon.
I know that if I were in a position to sue you, I'd be asking you how you KNEW that the glaze you had on the orange juice pitcher I bought was safe to use in this manner for storing orange juice. If you simply replied that 'the manufacturer said so', I'd be quoting back to you all of the the stuff the manufacturer says about not being able to control the end user's use of the product, how the results may vary based upon how the potter uses it, and so on. I'd then be looking at the efforts you went to in order to assure that the materials YOU chose to use on the product that I bought were appropriate.... stuff like like your education in the field, your testing procedures, and the records that you kept of those efforts.
Lawyer- "You mean that you have no idea of the actual chemical composition of the glaze you used on my client's pot which you sold to her? Yes or no?"
I'd also be trying to get the supplier/manufacturer to "roll over on you" by putting pressure on them.......... possibly by naming them jointly and severally in the original suit. If I can get them to say clearly that the end user is totally in control of the results of the use of their product, that it is your problem, that will be powerful in helping the plaintif to win the case. And the supplier has less to lose from turning on you than by fighting the other case. They agree to say what I want in court or in depositions for the case, and I drop them from being named in the suit. Simple bottom line math.
You are liable for the products you make. This is why part of your business plan should be to have product liability insurance.
PS: Hopefully Lawpots might be reading this and chime in with some accurate info. My info might be a bit dated and laws do change.