I would guess the optics are dirty(lenses and mirror) that could cause the picture quality to get worse over time. Cleaning them especially if it hasn't been done in awhile will do wonders for the picture as will doing the convergence.(it is a CRT Rear projection tv?) this may also help in selling it.
The Value for Rear proj. TV's is declining in this world of flat panel tv's and since yours is not even High def(or is it?)that makes it even a harder sell. look at your local craigslist for some idea of what like sets are going for to base your price on.
Honestly it will be a tough sell when people are selling bigger hi-def tv's for cheap(example in my area) http://portland.craigslist.org/clk/ele/1462797602.html
If you already have a buyer and don't want to do the work to get it in sell-able shape then my advise is $50-100 and that may be pushing it depending on your local market.
finally consider how much it will cost to dispose of it if no one buys it,that could be a factor in your decision.