2nd hand dvds

i brought a2nd hand dvd and it refuses to play on computer, what do i look out for in dodgy 2nd hand dvds when buying them,i didnt realise it was only acomputer dvd not a aordinary dvd player dvd, i have had this before how can i recognise a computer dvd as against an ordinary dvd player dvd

1: do not trust a burnt/written cd or dvd. ever
2: hold original disc into sunlight and check for markings, spots and scratches
3: examine booklet and case to see if they are originals or printouts
4: ask for disc to play or proof that disc works
5: ask for reciept and money back plan.