Bids for websites/startups on www.flippa.com, if you want to get your startup funded the best sites are angel.co and gust.com its not technically a bid but the outcome is similar.
Pitchswag.com would be a good place to check out as it connects entrepreneurs with investors.
I'm unsure if there are any platforms where investors can directly "bid" on equity with other investors. I do know that there are web platforms where accredited investors can buy equity.
www.wefunder.com is one of them.