I was wondering if anyone could tell me how much a 2012 250cc Honda Rebel would retail for?

Color is matte gray. EXCELLENT condition. Has 32 miles on it. Goes in to a heated winter storage every year from November-April. Gets maintenance done every year. Unfortunately for me, I injured my back after I purchased it back in December of 2012 and I haven't been able to barely ride it. I haven't been able to bring myself to sell it, but we'll be moving into a smaller space so I won't have a spot to put it unfortunately. I feel like the dealer gave me a low offer at $1,500. Wondering if anyone can help.

See what they're going for on eBay. In 2011, I bought a used 09 Rebel with 540 mi on it for $2750.
He probably is low balling you. But he doesn't want it for himself. He needs to sell it for a profit. You can try selling it yourself. And possibly make a little more.

The tires could be dry and hard or cracked and after seven years need to be replaced. What's the manufacturing date on the tire sidewalls? Also, what's the condition of the battery? What's the resting voltage? Will the bike start?

Dealers will always make a low offer - they have to make a profit and in the meantime they have to store it and possibly work on it to make sure it is in saleable condition.

If you were to offer it as a private sale you might easily make more - but there's always the risk that you do not attract a buyer.