I break even on the property's rental income / mortgage. 

Lots of breaks and short days in hot humid weather. 

I have been looking to buy some undeveloped land with the intention to break down and sell as lots or acres.

How should I deny this person without breaking the law or being too rude?

He says he has 5+ candidates lined up for me to consider this week, and is planning to pay me for Dec by end of week.The other 3 tenants have paid me for January 2020, but a second tenant (lets call him Jeff) stepped forward to ask if he can break the lease and move out because of this same really bad roommate.

You break the law when you make the offer and cannot afford the property or when you assign the contract for a profit.

At a $450K purchase price, we would barely break even assuming we could get financing. 

@Jerry Lucker I encourage people to try free options before they pay for anything that way they can see if it helps them before spending money, that is why I am a huge believer in free landlord associations, even inexpensive ones are probably fine, I just don't want people to spend thousands of dollars on some guru's "system" thinking that is the only way to break into real estate when it is advertised as an association and not a sales program.

Thank you @Ben Leybovich for the great podcast episode breaking this down so I can wrap my head around this concept: https://justaskbenwhy.com/2019/04/23/understanding-the-cap-rate/In SFH the measure of the market is going to be mainly what other comparable SFH are selling for. 

You'll need to run the math to understand at which point the appreciation you "give up" breaks even with the additional mortgage pay down and increased cash flow.