The people at Localbitcions that don't want to deal with taxes/IRS might be just misinformed or don't want to deal with the calculations involved in figuring out the profit, or in most cases they don't want to be tracked. - Taxes are paid based on earnings / income, selling BTC for a profit is income (the profit part is) or the service rendered that the BTC is paid for.
Over $9,999.99 in cash deposit or withdrawals in one day at one bank requires them to file a report as $10,000 is considered not normal by the government. The bank, at the bank employees discretion can file a report on any amount that they deem suspicious, that could be literally any amount that is out of the normal for your account, including you making multiple smaller deposits in an attempt to get around the reporting rules.
Wire transfers, checks or other monetary instruments (not cash or cash like) do not fall under the reporting rules as the cash that initially created them would or should have "been checked".
This isn't a tax thing, its to fight money-laundering. As for taxes when you exchange from one currency to another it could be considered a gain or payment to you and in some cases the exchange will report that as earnings.
Just a little addition to the reporting*
A bank employee can file a report for any reason they deem suspicious and the banks anti money laundering department must investigate every report, it is also against the law for the employee to tell you if they are filing this report. So if you piss off that bank teller they could be smiling back at you filing a report on your account and having your past transactions looked into. At no time do you as a bank customer fill out the report and in most cases it's done right after you leave or if there is a line shortly after you leave.
Also contrary to some people's beliefs doing these deposits at multiple branches has no effect on if a report is filed, if the teller is properly trained they will in most cases look at previous deposits for that day at other branches and do some math.