After the halving, many ASIC models will either become unprofitable or give very little profit. And in order for large companies to use all the power for maximum profit, they will turn off thousands of old ASIC models. And what will happen to these ASICs next? Will they be sold to other countries or disposed of?
If you want to buy a used ASIC now, don't do it. After halving there will be a big discount on such ASICs on the market.
Telegram seems to be the place most mining deals and business is done. You will often see large batches of old miners (sometimes in 100's, sometimes in 1000's) advertised for sale on Telegram groups, such as the following:
As long as nobody edits my post, I can confirm the above channels are genuine - however there are a lot of scammers on Telegram and it is important that you know what you are doing, and how to check / detect scammers.
Scott Offord manages a useful document here which lists genuine Telegram resources: