OK, my eyes are now bleeding from just reading all of this. As a business owner, I am going to give 2 cents worth of valuable advice to both parties.
Nifty, you trying to sell this machine, that IS not your property, is 100% illegal. Same with your so called mechanic's lien, you served no legal notice to the customer, it's toilet paper. You quoted the guy approx $2550, took over 2 years, but cannot show any additional costs quoted to the customer. You have caused some serious damage to your reputation. Your $800 offer made you look even worse.
Sticky, this is partially your fault for not following up with and demanding your machine be finished in a timely manner and asking for "where are we at" for costs. Nifty took as much time as you continued to allow. With that said, law in this case is on YOUR side. You actually may be due damages for him trying to sell your machine if this went to small claims.
Now, if the estimate was truly $2550, Nifty has all rights to exceed that estimate by 10% without having your permission. That puts this machine at $2805. Pay it , be done, BOTH SIDES.