Answering as a developer who also makes audio packs, so weigh it accordingly.
The thing I could never just buy was a loop that actually loops. Most packs ship a track that fades out or clicks at the seam, and the fix is not editing afterwards, it is how the file was rendered. Render past the loop end and sum the decay tail back into the start, then write sample accurate loop points into the file so the engine is not guessing. RPG Maker MV and MZ read LOOPSTART and LOOPLENGTH out of the OGG comments, Godot wants Loop Begin in seconds, Unity wants the import mode set. Very little ships with any of that, and it is the whole difference between usable and not.
Second gap: stingers and transitions written in the same key and instrument palette as the loops they sit between. They are usually sold separately in a different voice, so they never quite fit.
On the SFX side, one coherent UI family, confirm and cancel and error and hover that clearly belong together, rather than forty unrelated blips.