Here is the item again:
Watching the Home Shopping Network late one evening, Clarence saw handheld
televisions advertised for only $49.99, so he bought ten of
them,
in his lifetime, Clarence was certain that he would need them all.
The comma between them and in creates a comma splice. At this spot, there are two main clauses joined with a comma alone, making a major error. He bought ten of them is the first main clause. In his lifetime, Clarence was certain is the next main clause.
To fix this problem, you could put a period after them and capitalize the I for in.