Selling copies as originals is bad :)
The cue file
Cue files define the track organization of the CD. As iso files can't hold CDDA (audio) tracks, cue files are necessary to tell the burning software which files to use and how.
On the NeoGeo CD, the first track must always be the data track.
FILE "Game - Track 01.iso" BINARY TRACK 01 MODE1/2048 INDEX 01 00:00:00 FILE "Game - Track 02.wav" WAVE TRACK 02 AUDIO PREGAP 00:02:00 INDEX 01 00:00:00
Note: the 2 seconds pregap is important.
The iso file
The iso file must include the necessary files and respect the game CD structure.
The CD has to be burned in "mixed mode" or "game mode" if it contains CDDA tracks.
There is a higher chance of obtaining an unreadable disk if burned at high speeds due to track jitter. Burning average quality disks below 16x speed is recommended.
Some iso packs have their audio tracks encoded in MP3 to reduce the archive's size. The MP3 files have to be decoded to 44100Hz 16bit stereo WAV files with an audio file editor or converter. Some burning software can automatically do the conversion.
Nero and some versions of Nero Express should accept cue files and burn the iso and wav files right away.
In case of an incompatible burner or software issue, an alternative way consists of using a virtual CD drive (like Daemon Tools) to mount the cue file as a virtual drive, and use CloneCD to make a copy of it to the real drive (in "Game CD" mode).
It is possible to be able to read a burned game CD on a computer drive, but have troubles getting it recognized by a NeoGeo CD (no "PUSH START", no audio tracks...). This is because computer drives are more sensitive (CD-Rs are less reflective than pressed CDs) and of better quality.
Make sure that the CD contains the right data, by loading it in the Nebula emulator for example.
If the data is correct, try burning it at a lower speed, use another CD-R brand. Don't use CD-RWs, they very rarely work in the NeoGeo CD.
If nothing does it, the CD drive's laser diode might be too tired.