FBA's costs are high and that could impact the seller's profits.
FBA's costs are quite high and this reduces the seller's profits, especially for those who are new on the business front. The greatest advantage is that Amazon has a wider clientele and one's products are viewed by a larger group of customers.
The seller may not make as much profit on FBA but in the long run the seller is bound to expand his business and make more profit than he would have made selling on his own.
The high costs compensate for the trouble the seller has to take to do his own sales and marketing which itself would cost more for the seller. So I really do not think the costs are high when we compare it to the overhead costs involved in individual marketing and selling.