Was there a special name or term for people that sold fake relics to Christians on pilgrimage? Like selling swan feathers as angel feathers etc.
I do not know of any specific term for "people who sell fake relics", either applied by them or by their opponents. There are several general words that were used (in various languages) for conmen, or "shady" salesmen, not restricted to the religious context. In English, hawker, huckster, peddler and seller are examples (see their entries in the Middle English Dictionary for relevant citations, especially seller sense b, "one who accepts payment for bestowing a benefice, spiritual gift, an indulgence, etc., a simoniac"). Likewise, a counterfeiter was usually someone who forged coins, but could apply to any kind of fakery.
Meanwhile, there were religious occupations connected to money which were not necessarily illegitimate. These include the pardoner, well-known from Chaucer's Pardoner's Tale, and the quaestor, someone who collects for charitable religious causes. The Venerable Bede said (in a homily on John 2, quoted in the Decretum Gratiani, Causa 1, Q. 3, c.11 that "sellers of doves, who make the house of God into a house of trade" include those who use their rank in the church or spiritual gift to achieve any kind of human reward. 1 Many such peddlers of relics did hold some genuine church office; and they did not necessarily sell the relics, but would often use them to solicit donations. People like this are well-described with phrases like "a crooked quaestor".
Selling (real) relics is an example of simony, and people who do that may be called simoniacs or simonites. The name comes from Simon Magus (Acts 8), and was applied to a broad range of offences, including the sale of ecclesiastical offices, goods, or services. Other Biblical terms, somewhat rarer, are giezites (after Gehazi in 2 Kings 5, who claimed payment from Naaman in exchange for Elisha's gift of healing) and dove-sellers or money-changers (from Jesus throwing those people out of the Temple, in Matthew 21, Mark 11, Luke 19, and John 2).
The buyer and the seller are both simoniacs (see for example Peter Lombard's Sentences, Book 4, Distinction 25, Chapters 2-6). Money need not be involved: exchanging sacred things for favors, say, is also bad. Canon lawyers and theologians argued that the intent was important - since Simon Magus was condemned even though he had not actually succeeded in buying anything, and Gehazi was not the one who had actually healed Naaman. This argument is developed, for example, in Thomas Aquinas's Summa Theologica 2(2), Question 100, and his commentary on the Sentences. Therefore, even the sale of false relics would be covered by the category of simony. As far as I can see, canon law lacks specific snappy labels for different kinds of simony - there are just long technical phrases along the lines of "those who expose for sale the relics of saints" or "quaestores who misrepresent themselves" (Fourth Lateran Council, c.62).
1. Non solum uenditores sunt columbarum, et domum Dei faciunt domum negotiationis, qui sacros ordines, largiendo precium pecuniae, uel laudis, uel etiam honoris inquirunt; uerum hi quoque, qui gradum uel gratiam in ecclesia spiritualem, quam Domino largiente perceperunt, non simplici intentione, sed cuiuslibet humanae causa retributionis exercent.