We did all of the work on this property ourselves, but anticipate using more contractors, etc. in the future.

If the person you're using turns out to be good please send me their contact info so I can recommend them for future clients as well.

A great place to start is https://www.biggerpockets.com/meet which you can use to find like-minded people in your area.You'll find some helpful tips at https://www.biggerpockets.com/starthere.Thank you for your service and see you on future threads!

Once I start as an agent, I will be actively learning the market in general and for my own future purchase of a rental property.

Also, longer-term, the net proceeds from this deal will fund future deals and better cost of funds on those deals, which done right will help me continue to beat 401k returns. 

I recently listed a property in Haverhill with a HUGE upside on the cashflow potential,  future value add potential.

The construction is more likely to be hit and miss, resulting in nearly every street having at least one house under construction for the foreseeable future.

Will anyone even think about lending me money for more real estate in the near future, even if I have 20% down? 

will it impact my ability to make future loans ?

@Adam Schneider is right, we can't possibly advise you without seeing the numbers...My gut reaction is keep it and pull the equity out to use on future investments, but that's because personally I rarely sell any of my rentals.