I'm using shareasale.com as a merchant and I'm happy with them. You can always start an in house program and at the same time use shareasale.com
For the big affiliates that feel more comfortable working with an established network, they'll sign up using shareasale.com
For your in-house program, you can offer a slightly higher payout. This should entice some affiliates to sign up with your in house program. Even with the higher comission/payout, you will still pocket more money then using a third party solution such as shareasale.com
Give your potential affiliates an option