I remember in the 1970s my mum would put plaster of paris over an old bottle and then stick the seashells we found on to it. These days, I generally prefer to leave the shells on the beach and would prefer not to buy new seashells. Some countries now have strict restrictions on shell collecting to protect species which is a good thing. I think it was in Perth, Australia there was a sign on the beach asking people not to collect shells and to think of the sea creatures that may need them too for shelter, such as hermit crabs!
I also found this resin seashell candle holder. I have sold several paperweights, very similar to this, over the summer. They are very 1970s! Whenever they have appeared in our house, they are usually swiped by my two young daughters who are fascinated by them.
Apparently shell art originated in the 18th century and has its roots in sailors making valentines to send home to loved ones. They made shell arrangements in wooden octagonal glazed cases. Then, in the Victorian times exotic shells became popular and women used them to decorate boxes and mirrors.
If any of you happen to be in North Norfolk there are two shell displays which sound very interesting. The shell museum - http://www.shellmuseum.co.uk/ in Glandford houses the best shell exhibition in the UK and The Peter Coke gallery - http://www.sheringham-preservation.org.uk/ which houses an amazing collection of shell art.