Someone suggested this and I agree.
Break the units and denomination into two 'events'. So, we would have:
I Offer 100
My Offer-units $
My Terms 'plane ticket to BKK'
His Terms 150
My Offer 130
The event granularity works.
That brings the data structure down to 5 columns:
EventID (GUID or similar)
TransactionID (GUID or similar)
CommerceID (user defined)
TimeStamp (date time)
Edit: I forgot the EventType column, so 6 total.