Here at least the OP did not use the "I
inherited" or "I got these in a storage auction" or similar lines to cover over the fact the stamp may not be lawful property. Such is the reason I am always, and I do mean always, concerned about blinding giving information to criminals for reselling stolen and misappropriated material while those who reply think they are helping the poster and hobby for good purposes.
Note to Chasinfram: Now is the time to make a report to your local police, the APS and if you do not have the individual's current address, the IP Address from his post can be obtained by the police if necessary. Plus, once you file with your police, advise the police jurisdiction where the individual lives about you report to the police, APS and the address of the individual. Good luck.
Edit: I just saw your other thread. My advice still applies EXCEPT you will be needing a loss of over $20,000 to even get this started. Anything under that amount is like gone for your life time. If you have images which you send with your APS report those images can be used later.
If your stamps are insured, file in insurance claim. If it is a company which works with the APS that will be to you benefit.
Of course, If you have no reason to trust a foreign buyer will pay or your foreign buyer does not trust you to send the material, then the deal should not be done in a large group. One stamp at a time is necessary and if that produces a shipping cost that the buyer is unwilling to pay, you at least retain your stamps.
If you sell via Facebook, understand they have no protection policies. Other online platforms may offer some additional protection for you, but understand, you should never send what you can afford to lose. There is always a chance you will lose the material even with the most trustworthy of buyers. If you "buyer" is a scammer, understand they have more skill at scamming than you will have protecting yourself. Should you seem to know how to protect yourself, the scammer will just move on.