Where as many other midwest markets have low purchase price the rents are much lower. 

For example I am interested in a small storage complex in my area.

I've been doing that hands on since 2004 with great success and low risk.

It has tenants, I have had one month of vacancy since 2012, I would prefer to sell it without an agent, however I worry that there will be dozens of investors wanting to walk through the place, disturbing my tenants, making them antsy and the investors then low ball or otherwise do not generate reasonable offers.What is the best strategy for making sure the potential buyers are serious before letting them in? 

One way or another, and if nothing else, someone has to evaluate the property as to the probable cost to bring it to an after-repair-value that matches the surrounding market's fair market value for comparable properties - before ever offering to buy it at "such a ridiculously low price" (as the seller will term your offer at first), let alone, before offering to sell it at "wholesale" to your buyer (i.e., so far below fair market price at retail that the repairs can be covered as well as your profit and the flipper's profit in its eventual retail sale -- or 'buy-and-hold-to-rent-out'er's profit before they would expect to get it rented -- plus the title search, closing, and any tax costs related to your purchase from the seller (usually split somehow) - and finally, the title search, closing, and any tax cost split with whoever buys it from you.Now, that someone could be you (you do trust you and your repair-estimate abilities, don't you?)

I'm looking at rents of $4750, which are low.  

Just to play Devil's Advocate, are you confident in vacancy being that low?

I am almost certain my offer will be declined but since the house has been on the market for over 90 days in a region that most houses sell in less than a month I took a shot at a low ball offer.

As you can imagine the low fee listing companies want easy buyers and to do volume to make any kind of return.    

Here is a article on what you can do for a low appraisal:https://www.realtor.com/advice/buy/4-ways-to-deal-if-your-appraisal-comes-in-low/If you think the property will still be there in the next few weeks, I'd resubmit the exact same offer.