Still collision attacks would hardly be an issue, even if MD5 is chosen, for a simple reason; it isn't used for anything computer-related decision but rather to human decision.
One thing would be the scammer to say:
Go check my nick, saying the nick is John, on rating site A
- or -
Go check «something totally fuzzy and impossible to read that renders the same hash as John» on rating site A
Whereas for a computer: John and aiyho23#4072S""u3989hruiohefu9034 can be the very same thing (as it doesn't really understand what either one means), for a person it isn't. (Note: for the example I didn't compute either hashes, I don't know if that string gives out the same hash as John, may check but no need, I guess you can understand what I mean)
My understanding of your original proposal - and apologies if I've misunderstood - is that we avoid using "John". Alice has a good experience with John, so she uses your rating system. This results in a hash, H("John"), and a "score", 1. Bob then wonders if John is trustworthy, enters "John" into the rating system, the system looks up H("John"), and returns "1".
Mallory, who is malicious, can see all the hashes (we don't believe in "security through obscurity", so we have to assume she's able to get at the hashes, either by hacking the database, by bribing the operator, by sniffing network traffic, whatever). Mallory can then attack the hashes at her leisure until she finds a collision. It turns out (hypothetically) that "XyZ!2£" has the same MD5 hash as "John". Mallory then registers at a different Bitcoin site using the username "XyZ!2£".
Charlie then comes along, wonders if "XyZ!2£" is trustworthy, enters "XyZ!2£" into the rating system, the system hashes "XyZ!2£", gets the same hash as H("John"), and returns "1". Charlie - incorrectly - believes that Mallory is trustworthy.
Every hash algorithm is prone to birthday attacks, so the system is always going to vulnerable in the long-term. However, we know that attacks against MD5 exist that are far more efficient that birthday attacks - Mallory can find a collision for "John" far quicker with MD5 than she can with a more-resistant algorithm.
Again, apologies if I've misunderstood your proposed system, but as I understand it it is vulnerable to collision attacks (and I can't think of an easy way to avoid that), so it's essential to use a strong hash algorithm.