If you want to do that, just buy a 7 speed cassette. They are readily available. Running 7 of 8 cogs from an 8 speed cassette wouldn't work anyway because the first two cogs have built-in spacers.

As for buying a used cassette, I hope you didn't pay too much for it. Depending on how many miles it has, it will either slip when used with your existing chain or slip with a new chain. 7/8 speed cassettes can be had for less than $30. Buying used is false economy.