I've been selling cats on Marketplace since October 2012, and I looked at order transaction summary and counted 75 boxed cat sales, with no delivery issues. O problems out of 75 is a pretty good track rate
However, I also sell collars, and I have encountered delivery problems a few times when buyers order several collars, so I added this note to my collar listings: Buying multiple quantities of the same collar? Due to SL limitations for limited quantity items, ordering more than one collar at a time may result in partial delivery. If this occurs, please buy the collars one at a time, thanks!
There are definitely pros and cons to buying and selling on the Marketplace: fees to the seller, buyers can't see the kitty box directly before they buy, the Marketplace lags too just like SL, especially around Christmas! The positives are being able to shop online (which is enormously popular, I think there are a lot of people goofing off at work LOL), and also being able to narrow down a search using keywords and price ranges is a plus.
I will keep offering cats on Marketplace since there are customers who like to shop that way, but my favorite way to find a kitty for myself is still visiting the marketplaces in world, there is just no substitute for seeing a beautiful kitty "in the fur" and being able to zoom in on all the details!