Anderson lowers are the most abundant and cheaper AR lowers you can find. They are used for tons of builds, if you go to AR15.com. Most people are fine with them, the others don't like the rollmarks.

If you buy a lower, complete or stripped, it needs to be shipped to FFL for transfer if bought online.

Upper receivers do not have serial numbers. Only markings will be manufacturer and specs of barrel engraved onto barrel. These can be shipped directly to your house as these components are not considered firearms.

The lower receiver is pretty basic and standard through all the manufacturers. There are some variances such as NiB FCGs and heavier buffers. But usually you pay a little more for those. Though lower receiver builds can get pricey if you upgrade to aftermarket trigger groups, nicer buttstock and grips. The upper receiver can be the money pit since lots of the components will vary from manufacturer. The actual upper receiver can be bought cheap unless you go the route such as Noveske and BCM, you pay for the name. Barrels will greatly vary depending on how it's manufactured, caliber, twist rate, length, fluted, material, etc. BCGs will vary and depends on the type of coating and material. Upper parts kit will be pretty much standard. Handguards/rail system can be costly too. Gas block will vary depending on size. As you can see there are tons of upper receiver combinations.

I recommend checking out PSA. They have cheap lowers, uppers, and parts.

My current AR, Stag 3T-M





My new build, Spike's lower. I haven't decided what I want for the top end.