The point of no return for the “childhood’s end” ending for Bloodborne

The summoning range has since been updated via patches. As of patch 1.05, research found here indicates the following rules:

Let x be your level.

For upper limit of level you can summon:

x + 20% + 20

For lower limit of level you can summon:

x - 20% - 20

For upper limit of level you can be invaded by:

x + 10% + 10

For lower limit of level you can be invaded by:

x - 20% - 20

It should also be noted that if you are using passwords to summon a friend, this limit will be ignored and you can summon them regardless of their level. Players who are significantly higher than the summoner who are summoned this way will have their levels scaled down.

Here is a nifty calculator which does the math for you!

In answer to your question: No, there is not an equivalent system.

However, you can use insight, the ingame currency, to buy items.

Sources:
"Items in Bloodborne are found in game [...] through trade with insight, or from chests. "
This site mentions nothing about traders, and a wiki would have mentioned so.