I see a few things without knowing the entire situation.
1. He is getting your personal information somehow. This could be through a credit card, post office, mutual known people, shipping company, something that is common with your order information or personal information.
2. I would try changeing everything. Go to a friend's house or a relative's house and try shipping and ordering from that location under their ID. Have the packages/cues shipped there.
3. Where is his proof that these cues belong to him? If a bloke walked up to me and laid claim to one of my cues without written proof, he would not get my cues. It would not matter to me how honest they are if I know for a fact that I paid for the cues. Without written or some other form of proof, it is your word against his..... Don't get me wrong, if the proof was there, I would give the cue up, then, he and I would give the seller a visit.
4. He could have somebody watching you that you think to be a friend. If you are in the same business, I am sure you have common contacts. You can try to trap this person by putting information out there that is not true and see where it goes. I would say I am buying a mid level cue for so and so money and expect it sometime during the week. Say that you will have it at the local pub/pool hall on a certain day. Make sure you describe a good friend's cue that is unquestionably theirs. When the theif lays claim to the cue, you hand it back to your friend in front of a crowd. That should deface the bloke. Also, make sure to remember who was told what and when.