One thing to know is that an online TA may not be able to give you his or her best price online.
We use an online TA, though I call him. One year we decided to cruise to Alaska from Seatle. We found 3 cruises on his agency's website that fit what we wanted. I called to get pricing, and the one that was the most expensive on the website was actually the one he could sell for the lowest price. Cruise lines forbid lower prices from being posted, but they can be quoted over the phone.
For instance, a cruise line may allow discounting but not allow posting the discount. A TA may have reserved several cabins and be able to charge a lower fare, but no to post it online.