I wonder about the ROI of proactively mining for product.
IOW: I doubt it’s too awfully expensive to buy a mailing list of those within your target seller demographic and within X miles of where you live.
With bulk mail rates and a mail house, it wouldn’t be too awfully expensive to send out a mailer once every [insert time interval here] to all those addresses, boldly and simply (this is where your marketing chops come in) telling them who you are, what you buy, and how to get hold of you.
Waiting until there’s an estate sale, IMO, is waiting too long. Telling people there might be cash in their attics, basements, garages, and kids’ old bedrooms … may give you a first-mover advantage.
The competition might already be doing this, but … that wouldn’t inherently mean that you shouldn’t.
And I’m thinking USPS vs. email because your customer may skew younger and more tech-savvy, but your sources for vinyl … may be significantly older and still respond favorably to snail mail.
Maybe 
Somewhat separate …
I used to advise new entrepreneurs: broadly, you will need to either spend big on location or spend big on advertising. A thoughtful analysis will help you determine the degree to which walk-by traffic will convert to walk-in traffic, vs. whether you’re a “destination tenant.”
It’s a continuum. Where on that continuum do record stores fall.
There’s not much sense in paying for an A+, high-visibility, high-traffic location if people walking by will never become your customers.
Similarly, if you rent a corner of the local church basement and have zero visibility and no permissible signage, you’ll spend a fortune on advertising – maybe never enough to make the concern viable.