I found a great triplex with lots of potential but my realtor can find no sales comps within the last year in the area. It is in a section of the city that is separated from the rest. 

How will an appraiser evaluate the value in such a case? How can I analyze the ARV to know if I should move forward on this.

I called my insurance agent to get a quote and he suggest the rebuild value at $67K more than asking. But I'm estimating about $60 in reno. Even on the low end of rental rates it seems promising.

First deal rookie here. Your advice is much appreciated!!

There are always comps. You may just need to adjust a whole lot more than usual. If this is something you are serious about you can always pay a couple hundred for an appraisal. If the owner lets you and you don't mind risking a night out at movies and dinner. It's not a bad option.  If it's low, use it as a bargaining chip. If it's high, don't show it to them but make an offer.

@Adrienne Donner I agree with @Mike Cumbie - there are always comps!

Since this is your first deal I would say you may want to walk away. When there are no comps (especially coming from your realtor) that means you have no idea what the ARV will be. It could be higher than expected but most likely it will be lower than expected as banks will always error on the side of caution.

If you're really set on this deal then pay the ~$500 to get an appraisal or chat with a hard money lender (or whoever you're getting your financing through) and they may be able to tell you what they think. 

Best of luck!

Cameron's advise seem worth reading more than once.

In some other threads I have mentioned a comps nightmare we had in Louisiana recently where 15 appraisers refused to appraise a triplex because of no comps. The 16th was able to appraise and it is a work of art I should probably frame and hang on my wall. It was beautifully done and the work of a true professional. He indeed found comps and explained why they were used.

It seems logical, especially at the newbie stage and especially given the comps uncertainty, that when you calculate your numbers you include plenty of padding in your estimates. For rehab cost and time over-runs, for selling time over-runs (including more loan repayments) and for selling price surprises. 

Best of luck.

There are three ways to determine value. An appraiser will use at least two. Comparable sales, even if they have to go further back in time and further away from the subject property. Replacement costs. To build a similar structure using today's labor and materials costs per sq. ft. And the income approach. They will multiply the gross rents by a multiplier to estimate value.