Nermski, if you choose to go this route, I'd suggest you let your buyers know that there is an invisible pen mark in an inconspicuous part of the bag. Photograph it. Although chances are it may lower the value of the bag.
State in your listing if tyvek/security tag is tampered with or removed in any way, you will not accept a return. (Also include your return policy, shipping and handling for both domestic and international, etc.) Include photos of everything.
I would place a tyvek/security tag to a secure area such as the lock or strap, and on the tyvek/security tag, draw/write something that can't be easily replicated. Photograph that for your own record.
Before you pack your bag for shipment, wait for payment. I hear stories of so many new sellers shipping out things that haven't been paid for because they were pressured by their scamming buyers to hurry up and ship. Make sure PP tells you that you are "OK to ship". PP will also tell you what type of protection you have for: 1) items not received and 2) unauthorized transaction/payment.
This is clearly optional but with the stories about Chanel bags going missing, it's better to do it and have evidence than none at all. Have someone videotape you during the whole process of writing on the tyvek/security tag, putting it on the bag, putting the bag in the box, wrapping the box up, securing the package with tamper evident tape/security tape, driving to your local PO and handing the package over to a PO worker. Time stamp and all. Keep all your receipts.
Don't forget to fully insure and use signature confirmation. Make sure you use a trackable delivery method. HTH!