Seems like a good idea, but the accounts are waay too expensive. I have built up my activity/post count from the occasional post (maybe once a day), not worth .5 btc (~300$). Drop your prices a bit, and you will see some buyers. I would probably sell my account for a maximum of .1 btc, but I don't want to.
Thank you for your feedback.
As I said in the OP, I am willing to negotiate on all prices, and the worse I can do is tell you that your offer is too low.

You can easily ROI in under a month with a Senior account for 0.50 BTC on a signature campaign.
Not under a month. Currently the highest rank is hero that makes a difference in sig campaigns, and you would have to do 400 posts in a month to get the max of 0.48BTC, which is quite a bit.