As to a contact address, it is easy to find. Pick a recent show of significance which has a list of the dealers in attendance and find "The Classic Collector" which is the business name they used. If the Show listing included an address, you now have the contact information. One of the last shows I know they attended was WINEPEX in October 2022. That show lists the address and phone number. The address is good BUT THE PHONE NUMBER HAS A TYPO.
It was at that small show, put on by the Redwood Empire Collectors Club which attracts major dealers, where I purchased a world rarity (Finland) from Sergio. For two days we negotiated and then made the deal. He also lead me to the purchase from another dealer at the show (Kurt Schau), a Falkland Islands overprinted forerunner cover from South Georgia, one of five known of that type.
For three days the two of them, Sergio and Liane sat quietly at their table, little disturbed by pesky collectors. My world rarity was their first sale of the show. That just covered their show expenses, travel, room, table fees and meals with little if anything to spend on replacement stock. However the last day Sergio sold a collection of forgeries to a collector who specifically came to the show to view the collection. The net price was mid-five figures. One show, two sales and Sergio took in more money than any other dealer in the room. That was Sergio and Liane's quiet way which was driven by the quality of the stock they carried and knowledge possessed.
"Sergio will be missed" while accurate are words of woefully extreme understatement.