I know a number of the "quick home buyers" in the DC area. (In fact, I also offer a "We Buy Homes...Quick, All Cash" service. I'm not promoting it here, just mentioning that I know how it operates.)
Your property would be a good candidate for that process. The formula that they/we use in the DC area (and many other parts of the country) is: FMV (fair market value of the home if it were properly updated/fixed up) times 70% minus rehab costs equals the MAO (maximum allowable offer).
So let's say your Alexandria property is a cute little place in Del Ray that would sell for $700,000 in good condition. An investor would take 70% of that. So now we're at $490,000. Then the investor will subtract needed repairs and updates. Let's say it needs new carpets, paint, updated baths, and updated kitchen. Maybe $50,000 in rehab costs. The investor's MAO would be $440,000. That's the most he/she/they would pay. You might receive an offer of $399,000. Or $407,850. Or maybe very slightly higher.
It is possible to negotiate with investors, up to whatever their MAO is. And it'll be slightly different for different investors. Some may think the house would sell for $725,000 nicely fixed up. Others may calculate rehab costs at $55,000. Some now are using a 65% multiplier, rather than 70%.
Can the "We Buy Homes" people do what they say? Yes. Most of them have hard money lenders lined up, who've committed to loan the money so long as the deal works within the parameters described above. A few of the larger ones have their own sources of funds. And some mid-sized operations have private investors--less expensive than hard money lenders.
Anyhow, that's how you determine whether it's a good offer. And feel free to contact several companies. As I say, their offers can and will vary, at least slightly.
And, as Paul and Cindy suggest, have a Realtor do a CMA. Make sure you ask what the property is worth in its current condition and what it would be worth fixed up. The first number will tell you how much less an investor might give you than another buyer coming in off the MRIS. The second number will help you figure out how the investor is coming up with the MAO.
Hope that helps.