This too happened to me. I recently sold a gatcha full rare set for L3600. Lindens took the commission from marketplace and paid the lindens; I log off and return to negative linden balance.
I have raised a ticket about this, I received no explanation and it is hard for me to top it back up with the exchange rate as I live in NZ. They said I should have received an email but I never did and I haven't heard anything about my ticket either