It is good time as GPU based mining became unprofitable now

So, what models to look for on used market if you plan to use it in video editing?

First. Most frequent used cards in mining are - AMD RX570 (and RX470) 4GB/8GB, AMD RX580 (and RX480) 8GB, NVidia GTX 1060 3GB, NVidia GTX 1060 6GB.

All of them are very good in editing.

Note! Mining do not do anything horrible with the cards if rig had been maintained properly, do not fall for horror stories.

Second. Issues to look for:

Fans. Make sure to test fans at all speeds, at cold state and after warming. Lot of modern GPU fans are not too good after prolonged work.

You can also look for card with silent fan mode, where fan stops under low load (it is around 80-90% of editing work time).

Thermal pad leaks. Famous issue of some cards, looks like oily areas, or just slightly oily looking board parts

Contrary to this photo, it is usually located under cooler close to RAM or VRM area, and you can miss some of it. Such thing appear due to vertical position of the card in most mining rigs contrary to horizontal position in almost every computer case. Thing that is leaking is dielectric, so nothing bad must be happening.

If card works good, and discount is big, plus you are able to replace thermal pads you can buy such card.

Card under load. Use https://geeks3d.com/furmark/ and https://benchmark.unigine.com/superposition to check card under load for at least 30 minutes, it must not be crashes or visual artifacts, same as abnormally high temperatures. Can also check card model using https://www.techpowerup.com/gpuz/

Third. Cards to prefer - AMD RX580 8GB, NVidia GTX 1060 6GB, GTX 1070 Ti 8GB, GTX 1080 Ti 11Gb

Try to buy more powerful card with more memory. Video software, especially Davinci Resolve or advanced plugins is GPU memory hungry and become more so with each new release.

Mac/Hackintosh guys. Many Mac users buy AMD cards as external ones with special Thunderbolt case, yet it has lot of issues, check good articles and videos first. It is still way cheaper than pay for BM external solutions.

GTX 10xx series had good drivers and supported by recent MacOS versions (but Nvidia Drivers for Mojave 10.14 Not Available!). Just make sure to check Hackintosh special resources before doing anything.

If you are on super tight budged - your choices are good 1050 Ti 4GB models, GTX 1060 3GB and RX570 4GB (can be usually found for $120-150 used).

Cooling to prefer

Avoid so called turbine coolers on cards

Lower end cards are fine with just good large two coolers.

Best under moderate and low load are usually three cooler long cards (as they have bigger dissipation area and lower coolers speed)

Can check ASUS Strix line for such.

Fourth. Warranty. Many cards still have 2-3 year of warrant left, so you can buy such for more safety. In many countries outside US/EU you can see cheaper cards with no local warranty at all as they had been shipped from US/EU. If discount is big and seller provide his own small warranty you can try to risk.

Last. Newest RTX 20xx and AMD RX 590 cards provide very little for video editing and is considered non optimal generation due to little progress and price hikes. Most advantage is in very heavy tasks and usage of RTX 2080 Ti. So, for almost anything, including advanced grading you will be good using older card.

Note. Usually local sellers provide best deals and it is possible to check card, contrary to ebay. Check your country most popular C2C platform to buy used stuff.