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You sending a single mail to multiple recipients. The render function is only called once.

You have two possibilities:

Send multiple mails:

var promises = [];
for (var i = 0; i < fromUser.length; i++) {
    promises.push(new Promise(function(resolve, reject) {
        mail.sendMail({
            from: 'Website Support <help@domain.com>',
            to: fromUser[i].email,
            subject: 'Seller Matched',
            template: 'SellerMatch',
            context: {
                user: fromUser[i].Name,
                username: offer.owner.username,
                OfferName: offer.name,
                category: offer.category
            }
        }, function(err, info) {
            if (err) {
                reject(err)
            } else {
                resolve(info)
            }
        });
    }));
}
Promise.all(promises).then(function(infos) { cb(null, infos) }, function(err) { cb(err) });

I use Promises to collect all callbacks. If you can't use ES2015 you need to polyfill them or try an other solution.

Disadvantages: "To" contains only a single recipient. In error case you only get the first error and don't know which mails are sended and which not (and why).

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Render the Mail on the Client

I never tried this because it doesn't work on every client.