This cel is from episode one of the series.
Spike just wolfed down some food that was in
a lady's grocery bag.  (He was really hungry.)
Spike is so quick that it takes the lady a
little while to notice.  Then Spike smiles and
starts pulling out cigarette packs to give in
exchange.  So you're description is correct.  
It's a pretty funny moment.  

Faris Kalin, bard extraordinare
and Holy Knight of the KOME