I agree with Jason Chan, plus a few more factors.

The following are a list of criteria based on my own experience from my own launches and conversations with several big affiliates that have given me their opinion (I mean, humbly, not on the fact that I've sold thousands and thousands of my products, because I haven't, I'm a product creator trying to grow step-by-step).

Things potential affiliates look for BEFORE you launch
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- Obviously, they'll look at your product trying to ascertain if there may be demand for it. Depending on your niche look at what's selling right now; for instance in the IM niche there are trends, like some months everyone launches products about CPA, other months it seems to be the official "video marketing tools month" etc.

- Even if you have chosen a good sub-niche, they will also consider if it matches their list. I've had some affiliates told me they would be very glad to help me if I create a product related to [insert whatever topic here] because they have a list of thousands of people interested in that, but for the particular offer I was presenting them, they thought their list was not a good match, so they preferred not to promote. I understand them (many make a living out of this, so they can't risk "wasting" promotion slots in something that won't convert FOR THEIR LIST, and it's also better for the vendor as they won't send "useless" clicks that only drive conversions down.

- They will take a look at your sales copy.

- And also, they may be influenced by other affiliates you have on board. If X, Y and Z super-affiliates are going to promote, this must mean something, so they are more likely to get on board. It's kind of a snowball effect.

- The price tags, funnel, commissions. Even if you've not launched yet, obviously a product with a 7$ F/E at 100% commissions, OTO1 17$ at 100% and OTO2 at 47$ recurring at 50% is more attractive than just a single F/E offer with a 27$ price tag at 70% commission.

- And don't neglect the importance of affiliate contest. An afiliate could double his profit by promoting you offer if you give a juicy price to the person topping the leaderboard.

After you launch
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Once the product is live, obviously there are several figures they can check (and you should too, but the main ones are those pointed out by Jason:

- F/E Conversion rate. A F/E converting at 3% will get very few affiliates on board, unless it's a very high-ticket product. Depending on your price tag, you should aim at least at 10%, 20 or 30% being much better, of course, especially for very low-ticket prices.

- EPCs. This is the key metric, in the end. If an affiliate sends you 1000 clicks, and your EPC is 0,2, they'll make 20$. They won't even bother. If the EPC is 1.75$, they'll make 175$ for every 1000 clicks, so that's more juicy for them, especially if you add the contest to that.

As said, this is my experience based on conversations with medium - big affiliates.

I hope it helps!