I agree it would be interesting and perhaps helpful but most times people I sell to don't want pricing revealed and almost always don't want their identity revealed and most times I like to keep it private with my sales/purchases as well.
The issue I think is that sometimes it sets a benchmark which might represent the current reseller or end user market but many times there are variables that come into play with a sale that can skew what actual value is.
Most resellers can differentiate with a little research but many end users will try and use the price that works for them in a negotiation.
A big problem I find as well when selling to an end user or reseller, is if they know the price you paid they think that the price you paid for a domain has something to do with the value of the domain. It just doesn't in most cases. The value is when what you will sell it for and what I will pay for it align. And we all know that many factors can come into play when that finally happens.
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