I found this while browsing on an unrelated matter, and it is directed toward chemical suppliers/handlers in light of the tightening restrictions on pseudoephedrine/PPA/ephedrine as of 1996/1997 (which have now obviously been tightened). But the following is interesting and applicable to anyone who uses sources (whether non-scheduled ancillaries and supplies or scheduled hormones), and it is straight from the mouth of the DEA 12 years ago. It is also why posting sources is not allowed nor should be (even if they have flamboyant advertisements on a given site).
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Question: What constitutes a Suspicious Order and how can Suspicious Orders be easily recognized?

Answer: The DEA has frequently been asked for guidance on what constitutes a suspicious order. Repeated below are the items identified in the Chemical Handlers Manual, pending reprint. This material is useful and will be
supplemented with the results of the work of the Suspicious Orders Task Force created under the Methamphetamine Control Act.

The reporting of a suspicious order is required by the Controlled Substances Act and is of primary importance to DEA in limiting the availability of listed
chemicals in the illicit traffic. Each regulated person is most familiar with
its customers and the circumstances surrounding the orders it processes. The
chemical industry must use its best judgment in identifying suspicious orders.
The following are provided in order to assist the industry in identifying
suspicious orders:

1. An individual who desires to pay cash and wants to pick up the chemical(s).
2. An established customer who deviates from previous orders or ordering
methods.
3. A new customer or unfamiliar representative of an established customer
who orders listed chemicals.
4. A customer who has difficulty in pronouncing chemical names.
5. A customer who is vague about its firm's address, telephone number, and
reason for desiring a listed chemical.
6. A customer who wants a listed chemical shipped to a post office box or
address other than the usual business address.
7. A customer who prefers to pay by cashier's check, postal money order, etc.
8. A customer who will not furnish references or who is vague about furnishing
references for credit purposes.
9. A customer who desires listed chemicals for reasons at variance with
accepted legitimate industry practice.
10. A customer who is not a member of a trade, professional, or business
association.
11. A customer who furnishes false or suspicious addresses, telephone numbers,
or references.
12. A customer who refuses or is reluctant to establish a credit account or
provide purchase order information.
13. A customer whose communication either by telephone, mail, or other means is
not conducted or prepared in a professional business manner.
14. A customer who requests unusual methods or routes of shipment or who
provides unusual shipping, labeling or packaging instructions.
15. A customer who purchases unusual quantities or combinations of chemicals or
glassware in contrast with customary practice and usage.
16. A customer whose stated use of listed chemicals is incompatible with
destination country's commercial activities or consignee's line of business.
17. A customer with little or no business background information available.
18. A customer using a freight forwarder as ultimate consignee.
19. The use of intermediate consignee(s) whose location or business is
incompatible with the purported end user's nature of business or location.
20. Evasive responses to any questions, or responses that indicate a lack of
basic knowledge of the industry, or inability to supply information on
whether listed chemicals are for domestic use or export.

This list is not all inclusive. It is ultimately the responsibility of the
supplier to recognize a suspicious order.
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Given that this info is dated, my applicational take on it is that (A) you can obviously get yourself into hot water by being a dumb@ss, and that (B) you can get one of your favorite "sources" into hot water be being a dumb@ss as well, depending on the specifics of the situation.