We thought about buying a 36 foot slip a few years ago at 35k, but decided to rent instead. The current marina owners do not sell slips, but the original marina owners sold a few during the startup about 25 years ago. These original slip owners (or their survivors) have been reselling their slips for a bit more than was originally paid.
If we became LTL (life-time-lease) holders, we factored in other fees and surcharges. Depending upon slip size and seasonal or full-time liveaboards, the annual fee could be as high as $2,000-3,000. Apparently these fees cover marina expenses such as use of facilities, like heads, showers, laundry, parking, swimming pool, minimum utility usage, etc. We also considered the interest paid on our loan, if financed, and what that money could earn invested elsewhere - such as in appreciating real estate.
Since our summer rent is 4k and winter storage 1.5k, we didn't feel there was much of a savings and didn't want to limit ourselves to dock location or max vessel size. As it turned out, we switched slips three time for various reasons while moving through different boats. There really wasn't much appreciation in the market value of slips within the ten years we were renting - but obviously, the slips do sell for at least as much as they originally sold for.
If you're absolutely certain your boat size will not increase, or don't care about incompatible slip mates (read - party animals), then it's probably a wise choice.
Steve