What pandas function is most similar to the transpose procedure in SAS?
The pivot operator can be used to achieve the desired result. As per your example:
Define original data:
import pandas as pd
data = {'Txn_ID': [1, 1, 2, 2, 2, 3], 
        'Item_Desc': ['Apples', 'Milk ', 'Eggs', 'Flour', 'Salt', 'Eggs'],
        'Amt': [0.96, 2.10, 7.00, 2.20, 4.75, 3.50]
}

Create dataframe
df = pd.DataFrame.from_dict(data)

Transpose result using the .pivot operator
df_new = df.pivot(index = 'Txn_ID', columns = 'Item_Desc')

| Txn_ID | Apples | Eggs | Flour | Milk | Salt |
|--------|--------|------|-------|------|------|
| 1      | 0.96   | NaN  | NaN   | 2.10 | NaN  |
| 2      | NaN    | 7.00 | 2.20  | NaN  | 4.75 |
| 3      | NaN    | 3.50 | NaN   | NaN  | NaN  |