I'm planning to check the physical condition, ports, Windows activation, receipts, BIOS, hardware details and drive health. I also want to run benchmarks, stress tests, a memory diagnostic and a game, and ask about repairs, overclocking and mining.
Is that enough to feel reasonably confident buying from a seller with no return policy?
That's a pretty thorough list, but prioritize seeing the PC boot and run in person. Check that the CPU, graphics card, memory and drives match the listing, then run a game or graphics benchmark and make sure performance looks reasonable for those parts.
A video of earlier tests can help, but it doesn't replace seeing the actual machine work. That matters especially with an expensive graphics card, since a card can be physically present without working as expected.
Ask the seller to run the longer stress tests before you make the trip and show you the results. An hour of testing can reveal problems that a quick boot won't, and you won't have to spend the whole visit waiting for benchmarks.
Still do a shorter check in person. A recording is useful screening, not a guarantee of the machine's condition when you buy it.
Watch temperatures and stability during the tests, not just the final scores. Crashes, visual glitches, fans that don't spin, or signs of a leak are reasons to pause. Drive health information is worth checking too, although a clean report can't promise the drive won't fail later.
With no return policy, decide beforehand what you're willing to accept. Receipts and answers about repairs or mining are useful context, but they don't prove the current condition. Even a PC that passes your checks can develop a fault later.
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unfamiliar-filing:
Compare the benchmark result with results for similar hardware rather than expecting an exact score. Cooling and settings can affect it.