In addition to the risks to your collection, there are also risks to yourself when bringing in strangers around your animals and home, such as insurance should anything result.
Holding deposits are pretty standard practice in the industry, if it doesn't state, ask whether or not it is refundable. Either are acceptable, and its up to you whether you want to accept the risk of putting down a non-refundable deposit until you can view your snakes upon pickup.
As mentioned above, there can be an even greater deal of faith involved when it is not possible to view snakes before purchase. Such as importing animals overseas, or even just from province to province purchasing.
Its all your call, if you don't feel comfortable with those terms, then move on, if not go ahead, nothing is out of the ordinary in terms of standard industry practice