craigslist (website) is a great way to find used retail shelving.
It is free for dealers and liquidators to post ads, and they will use it.

If a dealer has a store fixture liquidation in progress, often the best way to get free, immediate advertising is from craigslist.  It will be their first advertising resource.

Take care though, it's craigslist.
Make sure the deal is legitmate.
Shelving and liquidations are a here today, gone tommorow kind of business.

As a buyer, you will want to check your local craigslist (website), and that of nearby cities/regions.

This website has state by state stategies for searching the best shipping lanes, but you will want to search all nearby cities for the best deal.Most ads at craigslist will be from dealers and liquidators, but many ads will be a "mom and pop" shop, just getting rid of their fixtures. They might not always understand the fair market value, and will set their price based on what they paid, minus a discount.

Search local and nearby regions, at craigslist, for a wide range of keywords: used gondola shelving; used lozier shelving; used retail shelving; used store shelving; Lozier shelving; Madix shelves; store shelves; retail shelves; store fixture liquidation; fixtures for sale...your average user will not necessarily know the correct name of the item they are selling.
Want to sell on craigslist?
Selling on craigslist isn't always easy.  It can get flooded with dealer ads, and go to the bottom of the list fast.

Here's some tips:
•List under either "general" or "business" category
•Use a variety of key words in your ad, so you show up in searches. Not just "gondola shelving", but also retail, grocery store, C-Store "shelves" and "store fixtures".
•Check that you didn't get "ghosted".  Sometimes criaglist filters out some ads...and doesn't even tell the lister!  Maybe the ad looked fishy to them, or like a duplicate of someone else's ad.
Whatever the reason, when you think you just listed at craigslist, you even got the confirmation that it's been listed...but the ad isn't live!!
You got ghosted.
•Avoid fraud.  It lives at craigslist. Deal locally. Check who you are dealing with. Only meet with a text record and/or in public places.
•Be detailed.  Write every single relevant detail you can think of.   Parts, pictures, inventory, description, condition, location, logistics...
Write it all down in your craigslist ad.  This will weed out useless calls, add credibility to your ad, and attract more serious buyers.
You can also try other local classifieds, like OfferUp (website) or Facebook Marketplace (website).