Sales of Personal Property and Proper Records

I have read the posts.  What is still unclear to me is what the IRS will consider as a proper record for determining the basis of any item sold.

 

Also, I'm unclear as to whether other sales of personal property can be combined with sales of personal property on Ebay.  I am selling things that have some value on Ebay.  Other items I'm selling on Craigslist and garage sales.   Can I combine all the sales to determine whether I have a gain or loss?