if you will be seilling using your own ebay id you will need to draw up 'contracts' with both people to show you are receiving all the money for the items 'on consignment' and you'll need to keep very acurate records of money in/ money out of your bank accounts, owise YOU will be liable for all that 'income' when it comes to tax time.

also, take time into consideration, who will be doing the descriptions, measurements, packing / posting etc.

who is responsible for any items that get damaged in your possession, does your household insurance give you any coverage??

selling building materials means pick ups and organizing times/dates etc. who will be responsible for that.

years ago I used to sell for someone, but I made him get his own ebay ID, paypal ac etc. and I had the passwords so all business for his company went thru HIS ebay ID and not mine.  I charged a flat hourly rate to list/sell.

whatever you decide, good luck with it, it's hard work but with good stock you can do well, with 'rubbish' you just waste your time.