Realistically, what's the most I could sell an iPhone 4 for?

Not to a store. Privately to a person. A store is probably only gonna give me 10 bucks.

Since it is an older phone, I would not pay more than $50 for an iPhone 16GB it with a charger. If it were in MINT condition, with case and charger, maybe $60-70. For the 32GB version, tops for me would be $110-20 bucks, again it is an older phone which is now getting less support, and no more OS upgrades, unless you have the 4S version.