I wonder if you are confusing Eurostar and Eurorail. The latter is a third-party aggregator and reseller that is seldom recommended here. I buy French tickets on the SNCF website, which redirects to "oui". I print bar-coded tickets at home, usually. It sounds (since you mention Versailles, which is not a long-distance rail destination, but a local RER commuter stop from Paris) like you might benefit from our host's general rail advice (blue menu top left.) His website is free and does not require registration to read his Travel Tips.
Presumably you know there are two adjacent stations in Lille, and which one has Eurostar. That is a premium service that is hugely cheaper when bought well in advance, yielding non-refundable, non-changeable tickets. The walk-up price is very high. Note that there are immigration formalities in both directions, which require up to an hour early-arrival at the station. For daytrips, you may use regular rail services, many of which don't require seat reservations, or a ticket for a specific train. (I mean, as opposed to the "Eurostar" product.)