I've done it once on a frame-up Motobécane build, and it turned out great.
But I did have a lot of help from the seller. He found me a perfect black/chrome Ishiwata fork, maybe a week after my purchase. I trusted him and he really came through. I ended up buying three bikes from him.
I bought another bike, a Miyata 912, from its original owner with a replacement Tange fork, and it's still in my lineup. I absolutely love it. He replaced/upgraded the original hi-ten fork right off the bat, though.

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With any used bike purchase, always run your hand over the tubes to ensure that no bulges are felt, especially in the front end. I also look for a perfect looking headset stack height.