First of all: what's your budget for this?
If you're going to hire a website development company to custom code a complex websites with shopping cart etc. then it wont be cheap for sure...
There are many good paid shopping cart scripts out there as well, and most of them (at least the good ones) will cost you from $200 and up.
Or you can also get free open source scripts like OSCommerce, ZenCart, OpenCart etc.
It's also possible to use a forum script like vBulletin and get a classifieds script (paid) to run a buy/sell site there..
It all depends on what you're going to use it for, and your starting budget..
It can be free or it can cost you thousands of dollars to get a site like that up and running...
I haven't used many shopping cart scripts myself, so I really don't have any recommendations..