PoD versions are well known on The Acaeum.  There are multiple legitimate sources for getting these, and as you mentioned, they can be a good alternative to purchasing originals, especially for those originals that demand a high price.

These PoD versions can also serve as a play copy so that a collectible original can be preserved.  The PoD versions will often contain the latest updates/corrections, and they use modern materials, which might be superior in some respects to the original materials.

There are also a variety of other formats available, for those that prefer and electronic version or wish to print their own copy.

Here are some sources, DMs Guild being one of them:

Dungeon Masters Guild -
DriveThruRPG.com - The Largest RPG Download Store!
RPGNow.com - The Leading Source for Indie RPGs

There are also some businesses that taken beloved classics and created updated versions, variants, or sequels to these originals.  Here are a couple of websites related to this:

Pacesetter Games & Simulations
Goodman Games|Your Source for Adventures

Perhaps other Acaeum members can add some additional websites.

Yes, occasionally shady sellers will attempt to pass PoD versions as originals on eBay.  When they do, Acaeum members will often post them here:

Shady Dealers

For unscrupulous sellers that attempt to sell copies of copyrighted materials, there is a another long-running thread here:

Report sellers who infringe copyrights!

Tszii wrote in Print on Demand and collectors:Do PoD copies have any tell tale signs? Bar codes for example?

I saw a B1 up on eBay the other day, advertised correctly.

Same cover as a normal 1st printing other than the UPC placed on the back cover.

This is the only one I've seen a picture of so far.

Almost all of them do have a new UPC or other notation / watermark indicating they are a reprint.  It's also sometimes clear, if you can get a good look at the printed product, due to the materials used.