You might try transforming the response. Heavily skewed data often benefits from the natural logarithm function. Alternatively, analyze the data with Fit Least Squares to determine the best power transformation:
- Select Analyze > Fit Model
- Select Openbid and click Y
- Select Item and click Add
- Click Run
- Click the red triangle next to Response and select Factor Profiling > Box Cox Y Transformation
Examine the plot of SSE versus lambda. If no transformation is helpful, the minimum SSE should be found near lambda = 1. Lambda = 0 is essentially the same as a log transformation. Click the red triangle next to Box Cox and select Save Best. Now repeat your analysis using Openbid X as the response.
See if this change helps meet the assumptions.