used engines often are sold because they are no longer performing. i wouldn't be surprised if the used engine no longer has any compression. i've come across several engines on ebay that while the seller says "good compression" it really doesn't have any compression and would necessitate the replacing of at least the ring.

i've been trying to get a compression meter but seems like few people sell them these days. i have bought about a half a dozen used but in "good shape" engines on ebay and only two have been bad so there is still hope :-)