Here is ESS's special pricing...
$8k, free install, no tax + $1000 for paint = $9k
You are selling for $6.8k + $1k for tune = $7.8k
Assuming someone lives close enough to ESS or one of the other sponsors that run similar deals, you are still saving the buyer over $1k. If the buyer does not live near one of the sponsors that offer free install and no tax, you are saving them the cost of install as well.
I think $6.8k is a good starting point for an OBO sale. Good luck with the sale. The black edition looks great.