If someone comes over to me asking to spare a cigarette and offers to pay for it, am I allowed to take the money?
Is it considered selling tobacco without having the proper license?
21 CFR 1140.3 defines "distributor" as
any person who furthers the distribution of a tobacco product, whether domestic or imported, at any point from the original place of manufacture to the person who sells or distributes the product to individuals for personal consumption.
You thus legally qualify as a distributor. Under the definition of "retailer", you would also be one:
any person who sells tobacco products to individuals for personal consumption
As a retailer, you may not sell cigarettes to a minor (21 CFR 1140.14(a)(1)). Under 21 CFR 1140.14(a)(4),
No retailer may break or otherwise open any cigarette or smokeless tobacco package to sell or distribute individual cigarettes or a number of unpackaged cigarettes that is smaller than the quantity in the minimum cigarette package size defined in § 1140.16(b), or any quantity of cigarette tobacco or smokeless tobacco that is smaller than the smallest package distributed by the manufacturer for individual consumer use;
so if you sell cigarettes, it has to be a whole pack, which by law is 20 smokes. You also may not give the cigarettes away, pursuant to (d)(1)
Except as provided in paragraph (d)(2) of this section, no manufacturer, distributor, or retailer may distribute or cause to be distributed any free samples of cigarettes, smokeless tobacco, or other tobacco products
where (d)(2) is about free samples of smokeless tobacco in a qualified adult-only facility.
So putting it all together, giving away a cigarette is against the law, since the law was so written. Selling less than a pack is against the law. Under federal law, acquisition of cigarettes is unregulated, so these regulations go to the seller and distributor.
Your state will also have its own rules about cigarettes which might prohibit the resale of whole packs without a license. Washington state for example requires a license to sell any form of tobacco, and there are various taxes on the sale of tobacco (RCW 82.24), but there is an obscure section that prohibits such resale. RCW 82.24.050(2) states that
A retailer may obtain cigarettes only from a wholesaler subject to the provisions of this chapter
and in case you thought you might not be a retailer, a retailer under Washington tobacco law is
every person, other than a wholesaler, who purchases, sells, offers for sale or distributes any one or more of the articles taxed herein, irrespective of quantity or amount, or the number of sales, and all persons operating under a retailer's registration certificate
I am not kidding. If you buy a pack of cigarettes, you are a retailer. This goes back at least to the 1935 laws (ch. 180 §83). Most likely they had in mind the context where a vendor purchases goods from a wholesaler and then re-sells them, but the definition as written would also classify ordinary consumers as "retailers".