Even though you might not like the fact that this employee has been shopping around it appears that there's a demand for his/her skills. That makes me wonder how hard it would be for you to replace this person.

Whilst making your decision you need to consider the following:
- Am I happy/satisfied with this worker as an employee, professional and person?
- How hard would it be to replace this person (calculate the time required for recruitment, induction, training and of course the fact that you'll probably have to say no to potential customers on the short term)

Based on these elements you'll likely find that it will make your life a lot easier to indeed give the raise. But perhaps you can create some additional conditions like increasing responsibility to bring in more sales, or coaching/training future new employees to balance out the higher pay.

Good luck and you know where to find me if you've got more questions!