The key trick is to find items that will have resonance with nostalgic feelings for the 1960's & 1970's and therefore appeal to the high-disposable income demographic of people who have recently retired or are in the prime of life.
Find items which used to be part of the everyday nuts & bolts of life but which haven't been seen for years. Items which will spark off instant
memories of a past which will suddenly seems so simple and idyllic.
For example, what about a Radio Times Leather Binder? Just the thing for Radio and TV Times in the days when 97% of the population watched the same programmes every week and used to discuss them in the Works canteen the following day. Cliff Michelmore, Bilko, I Love Lucy and the Arthur Haines Show. It was very crowded in that canteen as I recall.
Purchase said item and carefully take it home to savour, polish and prepare it for listing on eBay.
Don't under any circumstances leave it lying on a low coffee table unprotected from a rampant Petite Bassett Griffon Vendeen when you are out of the room.
...that's about it.