Questions about example sentences with, and the definition and usage of "Pcs"

A: PCs were expensive and people could only afford them about 30 years ago.
However, they are now necessities and one of the main sources of information.

A: Actually, it is grammatically correct. Great job for that. But don't repeat the words often once it is already understood from the context.

A: I would add a comma after "useful", so the sentence would be:

"PCs are useful, convenient gadgets."

A: What does the first 「～～」 correspond to? The manufacturer??

"The PCs from newegg.com can be purchased at the cheapest price if you buy them at newegg.com" .... >_<
あれあれあれ？

If you are referring to the manufacturer, perhaps:
"You can get the best price on X社's PCs if you buy them at ~~.com"

ex:
"You can get the best price on Dell PCs if you buy them at amazon.com"
"Dell PCs can be purchased most inexpensively if you buy them from amazon.com"

A: "with a 500 gb hard drive and 4 gb memory"