First it will work in any environment like mobile GPRS, EDGE etc, or different Operating systems or different hardware configurations.
It can accept all kind of Debit or Credit
cards, cheques from whole world! It may accept payment on delivery also.
Proper caparison should be there.
Some product may find in cheap at different special days in different Malls/shopping complex etc, so in website should be provide more cheaper than all others.
Delivery of purchased product in lesser days than other website. & it should be any where in world.
After purchase, if delivered item get any kind of trouble then what website do? means customer get assurance by website can care after purchase also.
security of entering all details regarding of payment should be best!
If in website, anyone sell old items also then check that item should not stolen from anybody. because i read about Rediff.com shopping someone sell old mobile phone & person who purchase it, get his original one which stalled few days ago before purchase from that site. So he went to police & Rediff say sorry for all.