There are several duplexes out there for sale and if you do it right, it can be financially beneficial for you.  

But if it’s a sale, some buyers might be okay with a few unfinished details, especially if you disclose them upfront.

It should include details like the purchase price, property description, and terms of sale.

I do generally agree with the sentiment of Austin and the other hosts that fundamentals will always win out in the long run, and this by nature is a long term industry, even the sales side I'm learning.  

I've been toying with the idea of adding an ISA to my sales business.

If you are looking to do real estate sales as well you may want to find a brokerage that hires for both at the same time, and check their credibility.  

Is there any creative way to avoid/reduce capital gains tax on the sale ?

This defers capital gains and depreciation recapture taxes on the sale of your fourplex.After converting it into your primary residence, you may qualify for the primary residence capital gains exclusion ($250K single/$500K married), but only for appreciation after the conversion; gains from the rental period remain taxable.

How many Due on Sale clauses have you actually seen enforced?

Jan we had 12 buys 10 sales.