Hello, welcome to the club
1) Chips are drilled in order to cancel them, this is mostly done by the casino. They are also destroyed by means of fire, hot stamping or buried in cement. There are many ways to destroy them, theres no particular method of choice.
2. Are drilled chips worth less then undrilled chips?
it depends to the buyer, a drilled chip has been used and has history in it, when a casino drills a chip, they drill ALL of them from that particular issue. Some may have escape this, from collectors that take them prior to being canceled, they could potentially be worth more than a drilled one, but again, it depends to the buyer. Same answer applies to the other question with regards to ease of sell or rarity.