All I can say is you are taking her words too litreally, just like she said "all girls" she also said "personal use" in the same sentence which is a contradiction.
I'm not a cop so I wont pretend like I know how it all works but I dont think either Lily is an undercover cop (because she is too young) or that her setting Quinn up to give her drugs can be used as actual evidence in court.
That was my entire point about her clearly not buying them for the girls but just saying she does. If you go through the dialogue Lily and Quinn have they are clearly both going for "plausible deniability". Lily is actively avoiding to say how much of which product she wants, merely going "500 bucks worth but I have absolutely no idea how much that would buy me" with Quinn playing along and going "I have no idea how much that is either, but 500 bucks seems like it's more than personal use." showing that she is a bit suspicious of she sheer amount Lily is asking for when this is the first time they are dealing with each other. That is when Lily presents the whole "It's personal use but it's for all the girls in the club." point in order to steer Quinn away from the suspicion that Lily might be buying her product to then sell it elsewhere, or to snitch on her.
That is why I initially went, quote "tbh. the thing is that I don't believe she actually is buying the drugs for the girls at the club." because I am not taking her words at face value.
She says she wants drugs from a reliable and cheap supplier -> Quinn is unreliable and overpricing her drugs to make up for the money that was stolen from her -> shit makes no sense
She asks for quite a lot of product from a dealer she has never dealt with before -> Quinn gets suspicious because that is a pretty big amount (any dealer would get suspicious if someone that has never even tested their product to check if it's even to their liking suddenly went for such amounts) -> only then does Lily come in with the convenient excuse that it's all personal use but for a pretty big group of people (all the girls in the club) -> Quinn is convinced and now wants to deal with her -> the problem here being that if you combine all that information it can be summarized as Lily saying "All the girls in the club take drugs but our old dealer is shit. Somehow out of all these girls working in the redlight district and taking drugs not a single one has heard of a different dealer we can switch over to, so our best option for 'reliable and cheap' drugs is an unreliable college girl selling drugs at double the going rate." -> shit makes no sense
Unless the assumption is that Lily is braindead and actively seeking out Quinn in order to get scammed by her prices, Lilys actions are unreasonable. If she wanted reliable and cheap pretty much anyone would be a better option than Quinn who is the opposite of both of those things. Likewise if the girls needed a new dealer then their own stripclub or any of the surrounding bars would be a better place to find a new dealer (or someone that can point them to a new dealer. like maybe one of their many many customers.) at than B&R. Hence why I think the girls know nothing about this and this is Lily buying 500 dollars worth for herself.
That poses the question why. She'd be putting herself at risk buying that much, when she literally lives in the same sorority mansion as her new dealer and could buy smaller amounts each time. No one enjoys sitting on illegal amounts of drugs when they could very easily avoid doing so. IF Lily is lying about the drugs being for the other strippers, then the only real reason I could see for why she asks for so much is in order to put Quinn in a situation where she has 500 dollars worth of weed (they talk about smoking and 500 bucks worth of crack would not cover all strippers, so I'm guessing they are talking weed) and the only real reasons for that that I can imagine is 1) to maybe steal it out of her room instead of actually buying or 2) to get her into legal trouble, since Quinn could never explain that amount being for her own personal use.
So the only split here is wether you think Lily is lying about who the drugs are for or not and we can agree to disagree on that. I am convinced she is lying, you seem to be convinced she is truthful. Only time will tell.