Do patent law firms also have services to help you license / sell your
patent to large entities?
Many licensing deals are negotiated and drafted by attorneys, but some are also negotiated by non-lawyers and just written up by attorneys. Licensing deals are also commonly negotiated by top executives of start up companies pitching their patents, or by designated senior executives in large companies who routinely seek to license patents held by others.
There is also a niche in the market for firms that buy patents at bargain basement prices not with an intent to use them, but with an intent to sue other firms that actually utilize technologies arguably covered by the patent (disparagingly called "patent trolls").
Often, the people who prosecuted the patent with the patent office are not the same individuals as the ones who handle licensing negotiations, but sometimes they are in the same law firm or are in two firms that regularly refer business to each other.
There is no single consistent business model that is used across the board with respect to this vertical integration (or lack thereof) of the patenting and licensing process work.