To purchase discontinued carriers, or used carriers sold by owner, there are a few sources I’m aware of (other than friends / family / garage sales).
This website requires a username and password, but is otherwise easy to access. It has been around much longer than Facebook (where the bulk of used baby carriers are sold). In addition to being a buy/sell/trade marketplace, it is a wealth of historical and educational information about baby carriers and babywearing. It is set up in an older forum style, and is a bit challenging to navigate.
There are many different “swap” groups on Facebook, specific for baby carriers sold by individual owners.
The two primary groups I use on Facebook, for buying / selling / trading (BST) of baby carriers, are the following:
The Babywearing Swap (all brands and styles) and
Babywearing on a Budget (all brands and styles, but carriers must be priced at $100 or less).
There are many brand-specific Facebook BST groups (such as Sakura Bloom BST or Didymos Love and Appreciation). The groups for more popular brands (such as Tula or Kinderpack) move quickly, and have hundreds of posts each day. Groups for less-popular brands move more slowly, perhaps one or two buy/sell/trade posts per day.
There are many local Facebook BST groups as well, where carriers can be purchased on a Craigslist-type marketplace, and transactions can occur in-person and with cash.
PayPal is the most popular method of payment for the Facebook BST groups. I recommend only using PayPal “Goods and Services” or Invoicing, since those give the buyer protection in the case of a transaction gone wrong. Paying with PayPal “friends and family” is not refundable, and the buyer cannot file a Paypal claim if the carrier turns out to be not as described, or the seller turns out to be fraudulent.
Sometimes Craigslist will have a great deals; other times carriers are overpriced compared to the Facebook swaps.
Be aware that some eBaby sellers, especially located in China, could be suspicious for selling Counterfeit carriers.
A few brands known to have counterfeits are Ergo Baby, Beco, and Freehand. Counterfeit carriers are not made to safety standards, and could use materials, threads, dyes, that aren’t baby-safe. They could be sewn improperly, or weaken quickly.
Counterfeits aren’t good for many reasons, but when carrying a child’s life in them, I wouldn’t risk it.
If you own a carrier you think could be counterfeit, contact the brand manufacturer with photos, and they can confirm for you.