It's not really reccomended to use the Trading Post to be like a SDB. They have taken measures to reduce it by having a lot lifespan of 2 weeks before it is returned to your inventory.
If you want to show off stuff but keep it for safekeeping, put it in your shop in an unbuyable state or place it in a gallery.
In my time on Neopets, I have seen major lost item glitches occur in the TP, in the SDB, and in the shops. So, I really don't know if anything is 100% safe anymore. I just throw stuff in my SDB and my shop and hope for the best.