Why do you want to buy or sell in different amounts? it would not change you anything because you are going to be paying the same fee as if you make one, or ten transactions.

I have been using localbitcoins to sell all my bitcoins for more than two years, i have never had any problems with them so far, it is one of the most serious exchanges that i have ever used, because their support is always replying (and this is the most important reason of why i like lbc)

And the money is always on your wallet, you do not need to pay miner fees because when you start a new order, you start it with the money that you have in your localbitcoins wallet, you do not need to deposit funds when you start X order, it just gets debited from your balance.