They are just for show, if you wanna be the "hey look what i got" guy then keep them, but if you dont care and wanna make some cash then sell...
The minipet prices cary from 1k(white mini)-100k+ectos(green)
Dependant on which year mini u get, 1st,2nd,3rd ect. The price usually goes up.
My advice is to sell, but if you already have abit of cash then theres not much point, unless your a money freak.