It is a used purchase to replace a leaking strut, when it arrived it heard a rattling sound inside the dust boot. Removing the dust boot shows the following in the photos, is that normal or do I have a defective unit?

 

My airstruts did not have any rattles of any kind when I purchased them and I got the 4 piece set off of eBay for $600 that is what I recommend for people too buy instead of Arnott. The only other airstruts that are worth too pay a premium are the factory bilstein because they are the only ones that have ECATs but those are $6k for a set off of SNG Barnett.

yes I do believe your airstrut is defective especially if it is leaking oil.

https://www.ebay.com/itm/16385750759...Bk9SR6z46quZZw

these are the ones I bought yes they are china airstruts but I have had no issues with mine and for one arnott strut you can have all 4