A lot of felons don't bother - what serial number is on a potentially stolen gun doesn't rise to the level of them illegally possessing one.
What is intended by removing the number is to make it as hard as possible to trace where it came from - more of an organizational issue. The reality is that if you did have the number, one cash transaction will break the chain and a lot of investigative time would be expended to discover who and where. Which is exactly what happens with a lot of stolen guns - arms traffickers don't keep books.
Despite the anti gunners intent to track every sale whereever it might lead, the reality is there's not really much to know if you do trace it. Consider the number of murders in Chicago, if that administration really wanted stolen guns off the street, it would already have been done - decades ago. Sting operations, mandatory jail for possession, etc and it would be cleaned up.
Historically - this all started with the switchblade ban in 1959. The response was - move to small caliber handguns. It was a test campaign to see if they could pull off a ban, and did. From there, they refined the process, and in 1968 they got the Gun Control Act passed, and only THEN was it required to serial number guns by the manufacturer. Up to then, it was more a requirement in large organizations just to correctly inventory them - like the Army.
Their next step we already know, every firearm transfer - even between family members - to be processed thru an FFL with a record of the transaction to file available to the ATF and if stolen then a criminal charge for losing it.
Which requires a serial number.
All that to explain why some by 80% firearms - just to put a stick in the spokes on their agenda. No serial number = freedom. The right of the people to keep and bear arms shall not be infringed. It is their very deliberate intent to use serial numbers to infringe our rights thru records, control, forcing us to buy a permit (= a poll tax) and eventually removing them from us.
Aint gonna work.