Finaly I found the solution.
I accidentally have TWO folders named Merchant Outbox in the Inventory main folder. You can see them turning on appropriate flags in Debug Options ( I use firestorm )
In such my case the problem is the merchant outbox window displays content of one folder while the viewer attepts to send items from another folder to marketplace.
I was unable to delete any of those two folders and stuck.
But I managed to send items ot marketplace. and that's how I did
0) make both Merchant Outbox folders visible in your Inventory and empty
1) drag any dummy object in the merchant outbox WINDOW. this step will mark the fake outbox
2) put in the OTHER(REAL) outbox the sub-folders you with to transfer to marketplace
3) click Send To Marketplace button
4) you get SUCCESS and your REAL outbox folder in inventory will become empty . this is good sign.
5) Make sure you got all the items loaded to marketplace
6) remove dummy object from outbox WINDOW (your FAKE outbox will become empty as well)
That worked for me.