I'm not defending Messicks prices but, as others have said, I think a lot depends on where you're located and how badly you need the part.
I live in PA, not far from a Messicks location. The shipping is a bit expensive, but all are overnight or second day delivery options.
I can usually get parts at my dealer for the same, or a bit cheaper than Messicks. The dealer is always my first choice, as long as they have the part in stock. When I need a part, it usually means a machine is down and I can't afford to wait a week or more. In that case, I'm willing to pay Messicks prices to get the part ASAP.
Case in point. Last Friday, I broke a wheel yoke on my Landpride mower. Dealer wanted $98 but it would take a week or more to get. By then, the grass would be a foot tall. Ordered from Messicks for $106 plus $30 shipping and had the part the next day. Yeah it cost over 30% more but to me, it was well worth it. If it were 100% or 200% more, that would be a different story.
I understand that not everyone has had this experience with Messicks, but for me anyway, I'm glad they're around when I need something quickly.