I was thinking that by giving a single builder rights to build on the land, I would get 1% of each contract price, something along those lines.

Just make sure you do you research on Manufactured guide lines and requirements. 

Are there electrical lines that interfere with the use of a  boom truck?

The other way I fund deals with none of my own money is using a private lender that I have years of experience and relationship with and/or using HELOC and other business lines of credit.

Best bet would be a “partnership” with a monied individual looking for a high risk/high return passive investment with you providing the “active” part.  

Look for your states offer to purchase contract online or contact a realtor to get a blank copy. review the agreement to be sure assignment is not expressly excluded. if there is nothing excluding assignment from the contract then writing the words "/And or Assigns" after the name or company's name on the buyer's line should be sufficient.Good luck!

For 1099 income, we will look at a 2 average of income using your NET NET income (bottom line income). 

Quote from @Griffin Malcolm: Open an LLC and apply for a small business line of credit from Key Bank.

If they have money and can compete on pricing, brokers will be the best bet.  

It sounds like you've absorbed a bunch of problems already and a new sewer line and the current basement situation will cost you way less than the $50K in fees to sell it.