I can shed some light on this. I'm involved in multiple collector communities and have come to understand the various ways of people.
- There are those who do not want to brag, or may feel ashamed if their buying actions are known. I'm sure you've gone to shows or sales where there are people buying things that just don't talk to anyone and are very quiet when dealing with a vendor.
- There are those who are just private people (maybe partially due to previous point) and go about their business by themselves. The most common type of person like this is the type that does not have an internet presence in their respective community. The usual "guess" as to what the actual online participation of a particular hobby sits around 5-15%. That 5% figure (I'd like to say at least 10% myself) comes from a manufacturer (not of cards mind you) as a response to a question about why they cater more to the common retail market than the hobbyist community.
- There are those that were ridiculed or fell they may be about what they own or buy, perhaps it does pertain to more "taboo" things like the extreme fleshlight example. They may also be trying to hide something from friends, family or co-workers.
- There are those who may find the ease of doing something online preferrable (rather than local transactions, or doing things on their own) but feel uneasy about the security of information. What with all the stories of places being hacked...
- There are those who may indeed be involved in shady practices.
To be a little more specific, these communities I mention earlier are collecting database websites. There are those who ask for an ability to make their collection private, and also those who would shy away from using such applications if that option were not present. The reasons why a person may want to do this likely varies greatly, but probably is some combination of the points I posted. I suspect this behaviour would also translate into making ebay feedback private, possibly for similar reasons.