Rare items don't come from special encounters. If you're talking about enemy encounters it's every hard to survive them at an early level, seeing as how you can enounter dozens of bad guys well armed.
FO3 there is no point in buying anything seeing as how you can loot everything you need off people. Hell with broken steel I can get a plasma rifle off a dead enclave at level 2 :bonk: Last couple of characters I had in FO3 did not buy one thing off anyone. Or need any repairs.
Super Mutants become relatively easy opponents towards the late/mid-game in the first Fallout game, with the Enclave replacing them in the second. In both you can easily get some of the most powerful weapons in the game off of their corpses (the Rocket Launcher and Gatling Laser come to mind). There are so many unique/event weapons in Fallout 3 that most players depend on them, and you obviously can't buy them anywhere in the game. However, in Fallout 1 and 2 the top tier weapons (Bozar, Plasma Rifle, etc) are for sale, yet they are also present in enemy encounters (but they do
not just randomly appear in caves and such in the wasteland). The only reason players would need to buy these weapons from merchants is if they were specifically avoiding encounters entirely. This high weapons cost in the early games is necessary to prevent players from easily skipping through most of the fights in the game.
What does the condition have to do with the initial buying/selling price other than bouncing up and down according to it?
Weapons are generally much cheaper in Fallout 3 because they end up seeing a lot less use for the player. The fact that they constantly degrade means you either need to find more copies of the weapon in the Wasteland for maintenance, or buy more from another merchant. Hence the initial disparity in price is made up for over the long run.