Check out https://www.balancedpayments.com/
They are made for marketplaces.

Airbnb CEO among others invested in them and they have some of the best pricing/payout fees.

Also some good info on http://www.collaborativeconsumption.com/2013/10/08/online-marketplaces-are-hard/
One of Balanced Payments co-founders is writing this blog series on marketplaces.

Look into stripe connect....much easier and you can likely just plug it in

In order to hold funds the minimum permissive structure would be to get licensed as an escrow company and setup a bank account specifically for the escrow funds that were not part of your operational funds. Most lawyers could provide such a service if it were sufficiently profitable to them. You would need to check into licensing for escrow companies in your local jurisdiction to be compliant. You should certainly not be trying to make any kind of interest or "money on the money" without proper licensing to do so.

I'd agree with Mikeal. I've also integrated with BalancedPayments for my startup BizeeBee. I went through a year long search before I chose them. It took awhile because Stripe and Balanced didn't exist when I was searching.

However, I don't know if Stripe or Balanced supports Canada yet. You might have better luck using BrainTree if you're looking for support in Canada.

Holding funds for users isn't the big issue. The issue is the reason why you need to hold funds, which comes down to two things:
1. Fraud - you want to make sure you've verified someone's identity before letting them withdraw a large sum. Your underwriter might also limit how much you can withdraw at a time e.g. PayPal has limits of $1000 initially.
2. Cost per transfer - most of the time it's not that expensive to do a transfer of funds to a bank account. However, if you find that in aggregate it's costing you a lot, then it makes sense to batch all the funds, and issue a transfer every few days or once a week.

Here is more info on the thought process I went through when picking a payment processor for my startup BizeeBee:
http://bizeebee.com/2012/04/bizeebees-process-picking-payments-processor/

Seems like an interesting venture.

When it comes to platforms and holding money there are strict regulatory & compliance requirements. As a platform operator you had rather focus on the core user experience and providing value to your users.

There are several payment platforms that will manage your transaction-holds experience ( for a fee) - PayPal, Stripe, wePay are a few I can recommend. These platforms take responsibility to manage the KYC, Risk, Regulatory/Compliance requirements for the transaction and associated holds experience.

Especially with online fraud continuing to be on an uptick it makes to rely on the experts. Happy to provide further advice if needed

Cheers

Holding funds in a two-sided market place can be industry specific. The magic word in these marketplace is "buy and sell freely" even if is just $10.
Besides if you do have any questions give me a call: https://clarity.fm/joy-brotonath