I want to buy onions in large quantity for storage so as to sell later. please i need advice from experts on how to go about it.

Onions should not be packed together. They should be spread out on cool dry places, not on the floor sha maybe on a bag and brought out under the sun once in a week to avoid spoiling.

Who farm the onions? Ones there is to much fertilizer during the farming u May encounter serious rout. Put ur onions in a well ventilated room on top of dry sand or dry hausa mat. spread it very well on the ground and be turning it once in two weeks gradually. Don't allow water or rain to touch it.

if the onions have just been harvested remember to cure them. Check out how to cure onions here http://www.gricd.com.ng/2017/10/09/how-to-harvest-onions-and-prevent-spoilage-by-curing/

Also, ensure that the onions have not already been previously damaged, injured, diseased or rotting, it will cause the spread of diseases and rotting among other onions. Try not to pack them together as well and spread them out in a shade