The 1848 Post Office Directory for Glasgow states:
Street Index:
182 Trongate, Spreull's Court
Gordon & Dunbar, messengers-at-arms
Auctioneers index:
Gordon & Dunbar, 182 Trongate
Surname index:
Dunbar, Charles, of Gordon & Dunbar, house, 182 Trongate
The other Dunbar entries do not mention a connection with this firm.
It seems that your Charles was the Dunbar, and that he lived "over the shop".
There is no mention of this firm in the 1849 Post Office Directory.