Both of them are correct. The difference between 完 and 掉 is 完 means something is saled and 掉 means something is sold out. It depends on what kind of situation you are.

我把衣服卖完了 = 我把所有的衣服都卖掉了 我把衣服卖掉了, this sentence is only correct, when it is in a proper context.

If you say "卖完",it means that you have sold all the clothes. And "卖掉" it means that you may sell one or two pieces of clothes, but you still have the clothes or no.

.If you say,I 卖完 these dresses,it means you have sold all the dresses.Do you still have question?

我把衣服卖掉-I sell the clothers. 我把衣服卖完-I sell all the clothers.