It's common for a broker to not accept SELL orders until shares are confirmed by the transfer agent or DTC, but new buys with cleared cash should be accepted.
Maybe I needed to clarify "tradable" on buy side versus sell side.
So TDA won't allow you to sell?
Now did a broker tell you this and did they use the phrase "front" the shares? As an ex-trading manager, I can tell you that doesn't sound correct, but every firm has their own policies to manage risks.