Perhaps the best, or cheapest, way to buy used retail shelving is from a store fixture liquidation sale.

Liquidate; to turn the shelving into money.

You buy the shelving and tear it out, right in the retail store where it sets.

For the liquidator, there are no tools, labor or shipping costs associated with your purchase.  These savings get passed on to you!

Getting retail store shelving from a liquidation isn't the same at every sale.
Sometime they will ship any order direct from the closing business.

Sometimes legal distinctions restrict the liquidator to perform on site sales only; perhaps a bankrupcy where creditors get a percentage of sales.
There is one common aspect when buying from a liquidation sale: the building must get "broom-swept" This means they HAVE TO remove all of the fixtures from the building...the landlords gonna want his building.  While experienced liquidators usually avoid getting stuck with fixtures, they will empty the building at any cost.

Shelving can be labor intensive, and it takes time to remove.  Often the best deals can be found from a liquator whose sale did not go well, and he is stuck with shelves he has to get rid of fast.  Or an out of state liquidator who doesn't want to pay $1800 for a truck to empty out his building.

Getting stuck with fixtures is an expense for liquidators, but realize one thing.  If the monetary difference is minimal, they might consider sledge-hammering, spraypainting and scrap the fixtures before they will take an insulting offer, or needlessly flood their market.
Liquidation Sale Buying Tips:

•Be prepared to work!   Liquidations are often selling fixtures where they sit.  That means when you make a purchase, you are responsible for removing it from the building.
Dismantling and packing 100 linear feet of shelving can be very labor intensive.  Be prepared with a work crew, pallets, shrink wrap, banding and a truck, or you might be in for a nightmare.
•Wait until the last minute! Liquidators often have a contractual obligation to clean out the building.  If they have to be "broom swept" soon, and the store is full of fixtures, they will usually lower their prices.  They don't want to get stuck paying for labor and shipping if they can sell out without touching the shelving.
•Don't wait until the last minute! (Yes, this contradicts our last buying tip)   If you need 22" inch shelves, and the store being liquidated only has 32 linear feet available , and the rest is 16", you might consider buying now, before someone else does.   Liquidators love to say, "Buy it now, before it's gone!"...they might want to scare you into an impulse purchase, but sometimes it is just good, sound advice.
•Haggle.   Store fixture liquidation prices are rarely set in stone.  Usually the liquidator has a little, or a lot, of breathing room when it comes to pricing the fixtures.
Ask about price breaks and quantity discounts.
Throw an offer out there. It might work.
If you notice they have a huge quantity of the some hard-to-sell-niche-product (30" tall green shelving, maybe), try to set your own price.
•Try catching flies with honey.  Remeber, the fixture seller often has control of the terms and prices of the sale.  If you are a pain in the butt, there might be "special" prices, just for you!  The process of buying used shelving can be logistically difficult.  If you are easy to deal and communicate with, the store fixture liquidator is in a position to reciprocate your kind nature with: better pricing on additional purchases; extra time in store or at a dock; use of equipment or staff.
•Know and understand all of the terms of sale.  Make sure you understand all of the terms of the sale.
  -Removal Date - When do you have to pick it up by? Hours of operation?
  -Equipment use - Can you use the liquidators loading docks, forklift, pallet jacks, pallets, and other things you might take for granted until it is too late? Remember, removal is your problem, not the liquidators.
  -Prices - Often sales will have a "buyers premium".  This is usually the fee that a liquidator or auctioneer will charge...it's thier cut and a common practice. It isn't subject to state sales tax, I believe.
  -Tax - Yes, you will be charged state sales tax. Write your congressperson.