1) As stated before, <SIGNATURE> - spare the extra $1.50 and get it on both outgoing/inbound mail;
2) Videos, ask/make videos of the gun shooting/gear/etc. if you are buying / shipping.
3) Insurance.. questionable, never had an issue, even when I opted-in for it.
4) Local deal? Try it out at a local airsoft place, and inspect it
5) Communication - can't get your parcel out today? Then let the person know, follow up constantly with ETA/Updates.
Basically do what you would want for them to you.
Extra work goes a long way, always better with a paper trail!