The agent calls it a "cash-cow block duplex."

Property details below but here are my questions:

1. The current ROI is listed as 8%. For any listing, would the seller/agent be able to provide accurate paperwork to prove validity of any listed ROI, cap rate, etc?

2. Per the listing, tenants are on month to month agreement so rent could be increased to market potential quickly. If I buy and increase their rent, current tenants could leave. May invite additional costs to obtain new tenants. What is the likely scenario in listings such as this. 

3. I tried to apply the 10% rule (dividing annual rental income by purchase price). 

Current rent of $11,100 makes it less than 10% = not a solid investment 

Potential rent lists $18,000 makes greater than 10% = a solid investment. 

Are there other exceptions to this rule to consider? Is this a good rule to go by? 

Property details here:

1110 W 8th St,

Sanford, FL 32771

Price: 139,900

Bedroom: 4
Bath: 2
Ext Const: Block
Built: 1985
Sqft: 1624
Ppsf: 86.14
Tax/Yr: 1852
Septic/Sewer: Sewer
HOA: None
County: SeminoleCurrent Rent: 11.100/yrCurrent Gross ROI: 8%

Potential Rent: 18,000/yr

Potential Gross ROI: 12.87%

Property Description: Investors looking for an income producing Duplex in Sanford should check this property out. Each side consists of a 2 bedroom / 1 bathroom 812 square foot unit. This will mark the third cheapest income property to sell In 32771 in the last 6 months. With move in ready single-family homes selling in this area at similar sq footage to each unit in the 100k to 115k range, it doesn't take long to see the value being offered here. This property has the same tenants in each side for over 4 years. Owner did not keep up with market rent. Tenants are both on a month to month agreement, so rent could be increased to market potential extremely quickly. At the current lease price, this property is still bringing in an 8% gross ROI. Update the rents to the market of $750 per unit, and this unit with no extra work will be yielding nearly a 13% gross ROI. Tenants pay all utilities, and each unit is on its own meter. If you are looking for an income producing property to add to your portfolio, this one is definitely worth a look.

Property features include; Block construction, good rental history, newer tile flooring throughout, updated kitchen cabinets, newer ceiling fans, all kitchen appliances included, no neighbors across the street, sturdy roof, updated landscaping, window AC units, central heat, convenient location to area playground and parks

Rent Rehab: None expected