Ah yes, I was going to post something like this soon. Basically, whenever you can, don't claim personal interest in the materials you are buying. In other words, say that you're picking it up for your friend/brother/roomate/girlfriend. All you need is a reason why the person can't pick it up for himself. Obviously, this isn't going to work at a chem supply place, but for essential oils or photography chems, it's a great way to avoid the need to educate yourself on the topic. It's easier and probably a little less suspicious if your mother/girlfriend/wife is interested in aromatherapy, not you.
"What are you buying this for?"
"Oh gee, I don't know. My brother is an amateur photographer and he wanted me to pick this up for him."
Oh yeah: don't give out any more information than asked, but don't be rude either. Just develop a good story and use it if needed.