Alot of great advice has been offered, and the same few themes run through all posts.
1. Have someone check them out with you. (i.e. your teacher is best or another member of pipe band as a second best option)
2. You may find decent sticks, and expect that you may need to spend a bit extra to replace what isn't good (bag/chanter/ect). Factor this into the cost.

However, don't be afraid to find what may be a good product when all the seller really wants to do is get rid of it without getting the full value of the GHB.
Good luck