Start conversation with buyer, or order the gig offered?

[klausvogstad]

I have been ordering some gigs for data entry jobs. Several charge x usd for y hours work according to the offer. But when contacting several sellers, many of them insist on different agreements of the type charge x usd for y pages.
So what is the common expectation ? Are gig offers just a starting point for negotiations, or should I expect the seller to actually stick to the proposed offer in the gig (i.e. x usd for y hours work) ?