Either: "There is a whole bunch of stuff presented on the market but only some of it is worth purchasing." or "There are a whole bunch of things presented on the market but only a few of them are worth purchasing." Right now, you're mixing number.

The market was packed with people selling heaps of different products, but only some are worth while actually buying. In NZ we have the slang word 'chocka' meaning heaps of...so we could say in complete slang 'the mall was chocka as with heaps of people and stuff to buy, but some of it is not worth buying.