I recently picked up four 8GB DDR4 sticks from the secondhand market. I ran Memtest and TM5 on them.
The first pair were Klevv DDR4-2666. One stick arrived with a capacitor missing from the top. It was a yellow-white component; the resistors around it are usually black, and these parts have four or eight legs. Even with the missing capacitor, the stick passed the test software without errors. That matches what some people online say: the part is a decoupling capacitor, probably similar in function to a capacitor in a speaker—cleaning and filtering current. Short-term use is likely fine; long-term use is a gamble. I did not want to burn my luck on it, so I returned both sticks together. I should add that this seller was a professional reseller of used parts.
The other seller was an individual. He was selling Klevv and ADATA memory rated at 3000MHz. Before buying, I checked my motherboard and confirmed it supported 3000. When the sticks arrived, one machine would not POST. I tried another computer and it booted. On boot, the MSI board asked whether hardware had changed and whether to update settings. I went into BIOS, changed the memory setting from Auto to XMP, and it would start—though it still kept prompting about the hardware change. In XMP mode the memory test produced errors. Looking around online, I found people saying XMP can be error-prone, especially in overclocking situations; in my case I was just trying to run the rated frequency.
I then looked at another computer. It had Klevv 3200 memory running downclocked to 2666, and testing there was clean. I repeated the same method on the problem machine, and it still errored. Single-stick testing narrowed it down: the ADATA stick was the bad one. I contacted the seller, sent photos, and we agreed that I would use it for about three days; if it proved unstable, I would return the faulty stick. The seller agreed. Running a virtual machine and playing Honor of Kings tended to crash when memory filled up. It was fine otherwise. I had not seen that before, so I suspected the memory—or possibly a compatibility issue. Klevv seemed to have better compatibility. Yesterday I returned both sellers' memory. One seller only took back the ADATA stick.
What I took away: with secondhand memory, individual sellers with good credit and plenty of clear photos are worth prioritizing. Some sellers use the resale function, and the listing photos are just the original merchant's images; in that case, ask for real photos of the actual item. Seller attitude matters too. If the title or details say sold as-is, no returns or exchanges, I would generally skip it. Professional used-parts sellers are still worth checking carefully—there can be good value there—but the quality is often not high, since the parts were not used by the seller. If a buyer returns something, they can resell it to someone less knowledgeable. They also cut prices decisively because their cost is low.