The decision to work with a rep really depends on several different factors. Without knowing what type of product you have, here are a few guidelines I can provide:
1. If the product can be placed specific to a certain retailer or type of retailer, it may be a good idea to work with a rep who has experience or relationships in that segment or company. For example, if you have a product specific to drug stores, there are rep firms within the industry who can help you get faster placement of your product because they know the merchandisers, objectives, and plan-o-gram schedules.
2. If the product is something that would work in the mass market, often times it is best to go it alone (also depending on how much sales experience you have) as it will be a lot less expensive. (Most reps charge anywhere between 3-15% of sales.)
3. It may be possible that your product is a good fit for various in-direct channels as well. ( i.e. Distributors who do merchandising for various retailer segments or display types such as clip-strip programs.) Again, this would depend on what type of product you are selling.
4. Lastly, in choosing to work with reps, you should understand that most of them are selling several different products and lines. In order to make sure you can help the rep be successful in placing your product (and focus on selling it as opposed to something else), they would need something very close to completion with sales materials that include MSRP, estimated annual units, product features, packaging/displays, etc.
I hope this information is helpful. If you have more specifics, I would be glad to provide additional suggestions.