I believe it is fair to assume that Valet service will be costlier for the user. So, if eBay makes the same profit on Valet as on direct selling, then eBay may not feel motivated to push through usability enhancements in direct selling, even though it is definitely beneficial for its users.
Of course, I am over-simplifying things, but I have been in a similar situation wherein the customers of our service started offering to pay us to help them use our service. Instead of accepting payments, we helped the users for free and took their problems as problems with the usability of our tool which we needed to fix. We believed that accepting paid service as a revenue channel would deviate us from making a good tool in the first place.
Of course, I am over-simplifying things, but I have been in a similar situation wherein the customers of our service started offering to pay us to help them use our service. Instead of accepting payments, we helped the users for free and took their problems as problems with the usability of our tool which we needed to fix. We believed that accepting paid service as a revenue channel would deviate us from making a good tool in the first place.