"Best" is a relative term. Talk to a good gunsmith.
If selling the gun is banned, then buying it before the ban is no problem. If owning the gun is banned then it does not matter when you bought it.
Have it checked over by a good gunsmith before buying.
As long as you go through a trusted website, buying anything online is safe, so make sure there are good reviews before jumping the gun and buying it.
Some can be $450 and some can be $1500 it is probably best to look on trusted gun websites before buying (Ive heard about this one called gun broker is pretty trusty but you might want to check on) your opinions if it trust worthy.
A good dealer. Your question is too broad to give a specific answer- that would depend on the type of gun. However, a good dealer will examine used guns for problems, wear or defects- and help guide you in your purchase.
no
Quality
Yes. If you lie on the 4473 and get caught.
Here's the best way to do it: Pay for it yourself, but have him fill out the Form 4473 at the shop you're buying it from.
Gun shop/ gun show
Gun shops, sporting good stores, the internet.
No money, no gun shop nearby, felony conviction, underage