In the US it's already legal to sell plasma, semen, eggs, bone marrow , breast milk, and rent wombs as a surrogate [0]. The legality of selling blood would not eliminate donations much like the ability to sell clothing does not eliminate clothing donation. Considering that there are negatives to selling blood (e.g. time, marginal risk, inconvenience, etc) I think some form of compensation is fair and would only increase the supply

[0] http://www.therichest.com/rich-list/10-body-parts-you-can-le...