Sealed is the only way to ensure that a record is in good condition. I have bought SO many records from 'reliable' sources, which look mint, but only play VG.
That is my experience also. At least half the used vinyl I buy lately is not satisfactory--it has too much wear (groove burn) to enjoy it. They can look clean and scratch-free but they do not play well. With sealed, at least the vinyl is untouched (even if it has other flaws). I know it won't be worn.
Life is too short for me to buy anything I will not use. If it's sealed, I open it. Although I wish I'd bought a dozen or so of those popular limited edition LPs a few years ago that now sell for 2x-3x the original MSRP. Gotta fund this crazy hobby somehow. ;)