The point here is that while share for sell transactions is low, in some periods it can increase, and it is good to cover this part of demand. (coinatmradar.com)
Without this margin, you would only be able to buy or sell transactions of $ 1,000 at a time. (eightcap.com)
Important to note here, that all operators who reported sell transaction average size mentioned it higher than average buy bitcoin transaction size. (coinatmradar.com)