It sounds like a great idea, but thieves might want to own the servers that store this data. On these servers will be a list of products and because of the receipts, very likely the physical address of where they were shipped to and how new they are.
Also, although https://www.ssllabs.com/ssltest/ does say the site supports 256bit AES, the unioncy server does not prefer it and under "handshake simulation" every current device will only negotiate a 128bit AES key.
Arguing about 256bit v.s 128bit AES on for TLS isn't really important though; the risks are all with the data-at-rest and stored on the unioncy servers.
In reading the privacy policy it appears that the product information will be used to provide selected and targeted advertising.
Also, although https://www.ssllabs.com/ssltest/ does say the site supports 256bit AES, the unioncy server does not prefer it and under "handshake simulation" every current device will only negotiate a 128bit AES key.
Arguing about 256bit v.s 128bit AES on for TLS isn't really important though; the risks are all with the data-at-rest and stored on the unioncy servers.
In reading the privacy policy it appears that the product information will be used to provide selected and targeted advertising.