There are many Membergate customers who do use 1ShoppingCart. You can set up a product page in Membergate. When they go to actually purchase the item, the link on the product page would bring them over to 1shoppingcart to finish off the purchase.
There are some issues to using 1shoppingcart, however. Since it is a separate, third-party solution all of your stats and orders are in a separate system and not contained within Membergate. Although if you have a lot of affiliates with 1shoppingcart, you may wish to stay with them as you cannot import your affiliates. It is important to note however, that in order for an affiliate to get credit for a new member on your site they would need to sign up for your Membergate affiliate program separately.
When you use the built in Membergate shopping cart system, everything is done from your site.