first thing first. Don't believe anything a seller says without verifying it. A seller is going to minimize expenses, exaggerate rents.... you need to do your own market analysis to find what rent is.... go on Craigslist or whatever is used in your area.... find 20 of the closest comps, and then figure that your subject property is below average.... don't forget to take into consideration the condition, the type of building, pets/no pets, location, length of lease... etc... this is something that will come easier with more experience, but I wouldn't take an 'average rent' from a website because there are too many factors. I've also seen landlords put crummy tenants in so that they can ask a little more.
Next, regarding the previous sale price.... we don't have those websites where I am, but I'd be real hesitant to use sale numbers since the deal isn't recorded... maybe it sold for more because there was owner financeing. Maybe it sold for less because the buyer had to throw out bad tenants... too many things you don't know. Look at what similar buildings sold for in the last 6 months, take that with a grain of salt because of the reasons I just stated, and then figure out what a building is worth.
When I'm looking at a building (and I'm looking at small multis right now), there are four things I look for, and in this order:
ROI. It doesn't matter if the seller is making a killing. If the price you can get it for gives you a good return on your money, then this is good.... I wan't 10% Cash on Cash, but I'll settle for more or less depending on other factors.
CAP rate. This is just a place to start. It's more effective the bigger the deal. However, with small multis, this is a flexible number to shoot for because it can be greatly affected by how well you manage tenants, or manage your manager. Look up on BP how to work CAP rates.
Money/Unit. Since I self-manage, I want $200 cash flow per unit. This is very hard to find in my area with the leverage I'm using, but it's a starting point. I'll go lower if my ROI is higher.
Opportunity to expand. I'm looking forward to the future: the last 4plex I bought has room for another 4plex on the lot and it is in a great location. When I feel confidant about the market... maybe 2 years from now, maybe 10.... I can build up that property and get a ton more out of it. Like all the other factors, I can be flexible about this if ROI and/or cash/unit are high enough.
I hope these thoughts help... let me know if you have any more specific questions.