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.


Question: The Probability of Catching Criminals
Answer: 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.