How feasible is it to sell a plane with an expiring annual?
I also know that a lot of people prefer to sell planes with fresh annuals.


I think the reality (from what i have seen) is that a a lot of people end up selling planes with a fresh annual. Essentially any smart buyer is going to want a pre-buy inspection and they are often willing to pay for them. In many cases pre-buy inspections are close to if not the same as an annual so the mechanic will often do the remaining work and sign it off as an annual resetting the clock. Any potential buyers are going to want to test fly the plane so its going to need to be air worthy. I would suspect the soon expiring annual has a small impact on price (in and around the cost of the annual which is comparatively low for a 150). 

You may find a buyer fast and unload the plane while its still airworthy, taking a bit of a  hit for the soon to expire annual or you may get it done and raise the price close to the cost of the annual.