If a person $A$ buys some (any) $10$ of the $1000$ items, then the probability that some other person $B$ buys the same set of $10$ items is 
$$\frac{1}{\binom{1000}{10}}.$$

For $N = 10^6$ people, and over $T = 100$ time units, the number of pairs of people is $T \binom{N}{2} = 100 \binom{10^6}{2}$.

So the expected number of occurrences of a pair of people buying the same set of items at the same time is
$$\frac{100 \binom{10^6}{2}}{\binom{1000}{10}} \approx \frac{100 \times 10^{12}/2}{2.6 \times 10^{23}} \approx \frac{2 \times 10^{13}}{10^{23}} \approx \frac{1}{5 \times 10^{9}},$$

which is small, but not as small as you calculated.

The Probability of Catching Criminals
Might sound silly for many of all but is this device's knob for the current for obtaining constant current or limiting the max current? Can I use this Dc power supply to obtain a constant current regardless of the resisyances? If not what would u prefer as an adjustable current source? Here is the device:
http://electronics123.net/amazon/datasheet/ps613u.pdf

When feeding a DC motor with this the motor spins faster when increasing the current knob. Im a bit confused if it is only for limiting purpose or not.
