So, I’m not saying these aren’t valuable minerals, but I’m a touch surprised you have an unsolicited offer. I’m seeing nothing nearby that usually draws offers: no producing wells in the section, no meaningful nearby permits or orders, nearest well in the last 5 years over 10 miles away. Was this a random letter you got in the mail or was it a connection of some sort? Did you recently inherit these mineral (sometimes buyers target new owners)? Am I missing something? 
Unless you’re dead-set on getting out of mineral ownership, this is not the most ideal time for your particular acreage to make a sale (but of course it’s your call!). Brokering a property of this size (at least, for the amount I’d ballpark your property at) is a bit like hiring a broker to sell a used car (as in, it’s a bit hard to do without it eating your profits and all the good ones are selling the expensive, fancy cars) but with even less visibility to know who the good guys are AND without Kelly Blue Book to tell you how much to sell if for yourself. I’d be very careful if you go that route. Everyone and their dog will try to convince you they’re doing you a favor taking these minerals off your hands “in such uncertain times.” There are auction houses who might be a better bet than a broker just due to more visibility of your asset, but heads-up those are also ~10% fees (which will be typical for your size parcel).
If you’re wanting to move forward, you’ll first need to have all your deeds, leases, and any other communication organized and ready to review. Make sure you know exactly what you own, that nothing is stuck unfinished in the court system or royalties sitting waiting for you to claim them anywhere (the buyer gets them if they’re unclaimed). Enlist help if you aren’t sure. There are resources cheaper than a title attorney who can help verify these things if there isn’t any thing complicated going on.
Since there isn’t any production on the land or comps in the area (and since comps are rarely helpful anyway), a right-sized appraisal is likely your best bet for determining target value. If this is your only acreage, the cost for that shouldn’t be too much to eat into the proceeds more than it’s worth.