I'll leave it to others to provide more technical answers.

Technically, the owner will not have title to the property to sell to you if the property is sold at the tax sale.

But they technically can.  

Your biggest challenge is not the technical implementation, but the ultimate impact your model would have on the industry, and thats why it won't succeed.

Technically you have to know the people. 

You can still qualify even if you are technically a dealer, developer, builder, etc. 

Yeah, well technically it is against federal law to put stuff in mailboxes except posted mail. 

Technically I can withdraw half the money from the brokerage (then the margin dips to 2:1) and buy real-estate.

Generally I'll try to get a bid from a 3rd party GC for the bigger jobs that are more technical to estimate - I am no contractor!

A land contract/contract for deed can also trigger the due on sale clause (especially since most people who have a contract sale will record a memorandum of contract on the deed, effectively "encumbering the deed with a sale.")If you want to get super duper technical, the land contract also is technically a subject to deal, although most subject to deals will usually have the deed transfer rather than being encumbered by a contract sale.Subject to means buying a property "subject to the existing liens, encumbrances, and loans."