Two people could register accounts, using their own names, addresses and credit cards, from different computers and Internet connections, with different ISPs from some geographical locations sufficiently far apart (e.g. different countries). Then, they would buy the same stuff and strip the meta-data from the files. Next, they hash their files and compare the hashes, by only telling each other half of the hash.
E.g., person 1 would tell person 2 that the first 32 characters of the sha-256 hash (represented as a hexadecimal string) was "47905a1f67a826d1ca5321e3473266d9". Person 2 tells person 1 that the remaining 32 characters are "7616ac096cb069098bcd2e12b2ed0c0d".
In our scenario, these two people have no interest in tricking each other - so they use the files that they bought and tell the truth when stating their part of the hash. The company from which the files were bought are assumed to not know that these two people are working together, since they might otherwise serve the same watermark to the two people.
Edit: In fact, these "two people" could even be just one guy who traveled to a foreign country, purchased a brand new computer, a pre-payed card, went to a library and registered with bogus details, bought some files. Then, s/he traveled to another country where s/he did the same thing (new computer, pre-payed card, went to some Internet café and used other, bogus, details to buy the same files) and compared the hashes as described above.
Edit 2: Or, s/he could order the pre-payed cards online, bought from two separate resellers using bitcoins, and use two separate VPN-s (also paid for with bitcoins) from a computer that was assumed to not leak any sensitive information (because the computer was bought for this purpose, and therefore never used for anything that could be tied to him/her, s/he used an Open Source Operating System, and s/he switched out the NIC and the USB-stick used to boot the computer between each time), with each VPN session being made from different places sufficiently away from your home, and at times sufficiently far apart (3-6 months, maybe).