Sales tax for the Delaware based LLC
Do we need to apply for sale tax number for the state of Delaware even though the number of sales in Delaware is negligible and the bank account is in Ohio so do we need to pay sales tax to Ohio as well?
Delaware has no sales tax, so you don't need to worry about it. As far as Ohio goes, if the only connection to Ohio is bank account I don't believe by itself it's strong enough nexus to require your company to collect/remit sales tax.
Keep in mind however that since you are using FBA you might be required to collect sales tax in states where FBA has presence (read this answer to better understand how it works).