Viator tours?
 

anyone heard of these? They are own4ed by trip advisor and in many cities---seem reasonable

answered you similar question here.
http://www.fodors.com/community/euro...s-442535-3.cfm

It is not a tour agency, it is a listing site just like Booking.com or VRBO.com etc
Like with TA do not believe the reviews.

http://www.viator.com/about/work-wit...ed-tour-guides

Viator's private tour guide website, tourguides.viator.com, offers an advanced booking service for qualified, local guides worldwide. Guides can leverage our sophisticated website to list their tours, license, photos, availability and payment details. Viator's private tour guide website offers advanced tools including real customer reviews, SMS booking notification, 100% online booking functions and more - making it easy for guides to generate bookings from travelers around the globe.

https://www.viator.com/terms-and-conditions
Private Tour Guides

33 Viator provides customers with information regarding Private Tour Guides in many destinations. Private tour guides are independent contractors and not agents or affiliates of Viator.
34 Customers are responsible for working directly with private tour guides on scheduling, meeting time / place, tour specifics and, where specifically noted, payment terms. Viator makes no representations or warranties of any kind regarding private tour guides or their services.
35 By booking a private tour guide, customers acknowledge and agree that, in addition to Viator's standard Terms and Conditions, neither Viator nor its agents are liable for any accidents, losses, injuries, damages, delays, expenses due to any action or inaction on the part of the guide or his/her services.
36 Viator's 'Worry-Free Shore Excursion' guarantee does not apply to private tour guides.

It pays to read the small print.

I once had to change arrangements with a Viator tour. Did it all through Viator, no problems.

As I said on your duplicate thread, viator puts guides on the viator website without the guide's knowledge or permission. I wouldn't use them.

We've used their sister company, Travel Toe, a couple of times without issue. They pretty much offer the same tours, however, have found Travel Toe a few dollars cheaper.