How do I add a Product to the Cart programmatically in Ubercart?
Is the node that users create is a valid product type node? In any case you can try this - http://api.lullabot.com/uc_cart_add_item 

uc_cart_add_item($nid, $qty = 1, $data = NULL, $cid = NULL, $msg = TRUE, $check_redirect = TRUE, $rebuild = TRUE)


Here $nid = nid of the product, $cid = cart id (also user id) - look at http://api.lullabot.com/uc_cart_get_id

Hope this helps.