I usually keep the receipt for warranty reasons, and I keep most of the stuff that came with the electronic device that I bought. That includes the box, manual, etc...
Having the box and all that came with it, gives you an edge when you plan to sell the item in the future.
If you plan to buy 2nd hand phones and there's no receipt it's still ok to buy that item as long as your conscience is clear and you feel the item is clean. =) Trust your instinct.
Katong gusto mo play safe nya walay receipt ang seller, pwede ra pud himoon consideration kung complete package pa ba, like box, manual etc.. usually stolen items kay ang device ra, dili makawat ang box, manual etc.. etc...