Current tenant is under lease until July and pays $750 p/m.  

I have a section 8 tenant in my rental property that has not signed a lease but I guess is still under the original lease umbrella.

You'll want to research the definition of real estate brokerage services in your state.If the definition includes language such as "Aid, attempt, or offer to aid in locating for purchase, rent or lease any real estate situated in [state]" then you have to be licensed to birddog. 

I keep seeing properties on MLS and similar sites in my area of Wichita KS where the asking price is 15-20% higher than what the local appraisal values the property.For instance , if the physical property has a value of 100k , but the asking price is 120k, does the the fact that there are existing leases in places warrant a 20% markup in the asking price?

Get copies of leases to make sure you're not walking into something unexpected.

Read the lease.

Obtain a working knowledge of the local rental market.If there is no lease on the property, but the local rental market is very strong, the lender may still consider the prospective rental income for offset purposes.

Hello,Before moving into this house, our landlord made us sign a lease for a year.

You could make an argument that your lease makes it the tenants responsibility, but there's no way this would be considered "routine". 

- Can I set up a fixed option amount within the lease and capture any overages if I do end up able to sell after lease term?