When tool is activated it gives everyone Strength instead of the player that clicked it why?

Because, the RemoteEvent is firing the whole server for each player, which gives everyone strength. To fix this, do it through the tool with a ServerScript, and you won't have to use RemoteEvents. Just a ServerScript.

You can keep the data for the strength, cash, and leaderstats. Just move everything else to a ServerScript inside the tool

Example:

ServerScript inside Tool

script.Parent.Activated:Connect(function()
    local character = script.Parent.Parent
    local player = game:GetService("Players"):GetPlayerFromCharacter(character)
    local leaderstats = player:WaitForChild("leaderstats")
    local Strength = leaderstats:WaitForChild("Strength")

    Strength.Value = Strength.Value + 1
end)

And then it's basically the same script that you have for the server script in ServerScriptService, just remove the strength part, as that's should be in the sword or whatever you'll be doing with this.

ServerScript inside ServerScriptService

game.Players.PlayerAdded:Connect(function(player)
    local sell = game:WaitForChild("ReplicatedStorage"):FindFirstChild("Sell")

    local leaderstats = Instance.new("Folder")
    leaderstats.Parent = player
    leaderstats.Name = "leaderstats"

    local Strength = Instance.new("IntValue")
    Strength.Name = "Strength"
    Strength.Parent = leaderstats

    local Cash = Instance.new("IntValue")
    Cash.Name = "Cash"
    Cash.Parent = leaderstats

    sell.OnServerEvent:Connect(function(player)
        Cash.Value = Strength.Value + Cash.Value
        Strength.Value = 0
    end)

    player.CharacterAppearanceLoaded:Connect(function(character)
        local humaniod = character.Humanoid
        humaniod:WaitForChild("BodyDepthScale").Value = .5 + (Strength.Value / 100)
        humaniod:WaitForChild("BodyHeightScale").Value = .5 + (Strength.Value / 100)
        humaniod:WaitForChild("BodyWidthScale").Value = .5 + (Strength.Value / 100)
        humaniod:WaitForChild("HeadScale").Value = .5 + (Strength.Value / 100)
        humaniod.WalkSpeed = 16 + (Strength.Value / 10)
        humaniod.MaxHealth = 100 + (Strength.Value / 10)

        Strength:GetPropertyChangedSignal("Value"):Connect(function(value)
            humaniod:WaitForChild("BodyDepthScale").Value = .5 + (value/ 100)
            humaniod:WaitForChild("BodyHeightScale").Value = .5 + (value/ 100)
            humaniod:WaitForChild("BodyWidthScale").Value = .5 + (value/ 100)
            humaniod:WaitForChild("HeadScale").Value = .5 + (value/ 100)
            humaniod.WalkSpeed = 16 + (value/ 10)
            humaniod.MaxHealth = 100 + (value/ 10)
        end)
    end)
end)

I made a little bit of tweaks, and just for your information. You cannot use game.Players.LocalPlayer inside of ServerScripts.

If you need more information on RemoteEvents, I answered a post about RemoteEvents here. Hope this helps!

If this worked for you, let me know by selecting this as the answer. If it didn't comment down below what went wrong and I'll be glad to help!

Because you're handling the OnServerEvent on the same script that handles CharacterAdded. Now this would not be an issue, but you make it set Cash.Value. Cash was declared above as the character that was added. This will happen everytime someone joins. You can fix this by changing your script:

local player = game.Players.LocalPlayer
local lift = game.ReplicatedStorage.Lift
local sell = game.ReplicatedStorage.Sell

game.Players.PlayerAdded:Connect(function(player)

    local leaderstats = Instance.new("Folder")
    leaderstats.Parent = player
    leaderstats.Name = "leaderstats"

    local Strength = Instance.new("NumberValue")
    Strength.Name = "Strength"
    Strength.Parent = leaderstats

    local Cash = Instance.new("NumberValue")
    Cash.Name = "Cash"
    Cash.Parent = leaderstats

    lift.OnServerEvent:Connect(function(plr)
    local leaderstats, strength, cash = plr:WaitForChild("leaderstats"), leaderstats:WaitForChild("Strength"), leaderstats:WaitForChild("Cash"); -- Get the player who clicked and their stats
        strength.Value = strength.Value + 1
    end)

    sell.OnServerEvent:Connect(function(plr)
    local leaderstats, strength, cash = plr:WaitForChild("leaderstats"), leaderstats:WaitForChild("Strength"), leaderstats:WaitForChild("Cash"); -- Same thing here
        cash.Value = strength.Value + cash.Value
        strength.Value = 0
    end)

    player.CharacterAppearanceLoaded:Connect(function(character)
        local humaniod = character.Humanoid
    local plr = game.Players:GetPlayerFromCharacter(character);
    local leaderstats, strength, cash = plr:WaitForChild("leaderstats"), leaderstats:WaitForChild("Strength"), leaderstats:WaitForChild("Cash");
        humaniod:WaitForChild("BodyDepthScale").Value = .5 + (strength.Value / 100)
        humaniod:WaitForChild("BodyHeightScale").Value = .5 + (strength.Value / 100)
        humaniod:WaitForChild("BodyWidthScale").Value = .5 + (strength.Value / 100)
        humaniod:WaitForChild("HeadScale").Value = .5 + (strength.Value / 100)
        humaniod.WalkSpeed = 16 + (strength.Value / 10)
        humaniod.MaxHealth = 100 + (strength.Value / 10)

        strength:GetPropertyChangedSignal("Value"):Connect(function()
            humaniod:WaitForChild("BodyDepthScale").Value = .5 + (strength.Value / 100)
            humaniod:WaitForChild("BodyHeightScale").Value = .5 + (strength.Value / 100)
            humaniod:WaitForChild("BodyWidthScale").Value = .5 + (strength.Value / 100)
            humaniod:WaitForChild("HeadScale").Value = .5 + (strength.Value / 100)
            humaniod.WalkSpeed = 16 + (strength.Value / 10)
            humaniod.MaxHealth = 100 + (strength.Value / 10)
        end)
    end)
end)