Is it possible to buy all the skills?

As killing enemies earns experience points, which eventually lead to a level up and a skill point reward, it is a safe bet that you can earn enough skill points to buy all the available skills in the game, by killing enemies and completing quests.

This is reported on the Twinfinite site also.

it looks like the max level cap is level 50. By this point, you’ll
have already earned enough skill points to unlock all of the available
Skills.

..

Upon reaching level 50, you’ll also earn a bronze trophy. It’s the
final level-related trophy, so it’s a good sign that this is the max
level cap in the game.

Ref: twinfinite.net Max level cap

The top row of skills all cost 1 skill point, the whole row costs 9 points. The second row skills cost 2 points, with a total of 18, and the last two rows of skills all cost 3 points each, totalling 54 point for both rows.

So in all you’ll need to earn 81 skill points to buy all the skills. Might be 80 in total, as I think Silent Strike is given for free.

As mentioned by @Shaz in the comments, there is also a trophy for unlocking all the skills.