first off, the guy selling those cables is BSing. the cables won't do squat in making the transfer rate faster, appart from either being a 40-wire or 80-wire IDE cable. most rounded cables are 80-wire, so they'll get the faster speeds.
secondly, IDE cables will never go faster than ATA133, and most hard drives don't run at over ATA100. and even if they did, it would be pretty useless, because the drives themselves don't even get ATA100, and only just use ATA66
just don't believe what people say about their products on ebay. they're probabbly exaggerating.