you can do this:
1) buy the domain so nobody else gets it. its just about 10 bucks/year.
2) develop content around the long tail keywords on your main site. it is already established and know to the SEs so gets spidered faster.
3) also develop content around the longtail keyword domain and put this on your main domain, but maybe use it to redirect to your merchant or shop software.
Hope i made myself clear - still too early in the morning