Brandon,
it is true if you had a deal it would be some much easy to build a cash buyers list and the truth is that you can only grow this over time. every call you recieve, email responding to a houses for sale etc.. those should be added in to a list.
If you want to build a list here is how i started and it is what I do till this day. I created an spreadsheet that I half a printed paper copy of. It has 4 simple lines, name, phone number, email & notes(to write criteria, company name alternitive etc...) I drive around and every time I see a for rent, for lease, we buy houses sign. I write down the info and call later in the day i talk to them if they are buyers then the go on my buyers list. I also got to craigslist and I do the same thing. Most landlords are buyers. The others are seller leads!
But if you want to go the ad route. Go to Jotform.com(no affiliation)create a buyer list form. ask for name, number and emails.
right a simple ads with simple titles that speak to investor/buyers as if you were selling a house or offering a service.
Investor Special- High Equity Rental Properties Available for Sale etc......
Then in the Body Keep it simple, We are (ABC Company) and we specialize in selling investment properties in the (XYZ Location). You can have your choice of long term rentals or short terms flips.
If interest please provide your information and we will send you a list of investment properties in your area. (link to Jotform link here)
Use this with different titles and different ads with keywords so that any seach that is done, this will show up