The exact same "argument", such as it is, could be made against adoption...
That's true --- but only to a limited extent. In many adoptions the adoptee can eventually find out who his genetic parents are (for instance, my Russian son can easily see on his legal adoption papers who and where his birth parents are in Russia.) There are a number of organized search groups in the US that help adult adoptees locate their natural parents as well.
Another, more important point is that gamete vendors will generate many more children than most birth parents will. Sperm vendors especially can beget dozens of children. This (like male promiscuity) dramatically increases the number of half-siblings out there, thereby increasing the chances of inadvertent incest and all its accompanying troubles.
If that were the case --- if a child were begotted for the purpose of being bought, sold, traded, or adopted out --- that would be immoral as well.
As the case stands, adoption is a matter of responding to tne needs (indeed the rights) of a child who was brought into this world poorly-provided-for. There the child is; he needs parents; the adptive mother and father step in as needed.
But vendor-gamete conception, the child is being deliberately brought into existence with the expectation (in fact the hope) that he will be permanently alienated from at least one of his genetic parents.
His very existence is predicated on the idea that his birthrights are transferrable. From the point of view of the child's own dignity, this is wrong.