'Collector' and 'metalhead' can be defined in various ways, but IMO it is very easy to be both at once. Most metalheads (i.e., people who enjoy metal music) end up obtaining personal copies, on some format or another, of a significant amount of metal music. This is simply b/c metal music is not heard often on traditional music outlets (TV and radio). Thus, in order to hear it, you have to get it for yourself. Thus, they can be considered collectors in a general sense. This dynamic is changing somewhat since people can now easily access music through new outlets like Pandora, Youtube, etc, where they can hear non-mainstream music without having to purchase it. And some metalheads will never own many albums, cds, mp3s, etc due to financial and space constraints. So not every metalhead collects, but I think it's pretty common to be both.
If you use the word 'collector' in the more excclusive sense of someone owning thousands of items, or spending thousands of dollars on metal, or someone tracking down expensive obscurities, well, only a small number of metalheads are collectors in that sense.
Let's look at it the other way too: how many people collect metal records who are not metalheads? There always seem to be lots of accusations/rumors of 'poseurs' buying up metal records and just treating them like stocks- buy low, sell high, don't even know, or care, what the product is. But how many people are REALLY doing that? I may be wrong, but I think people overestimate how many 'metal stock brokers' are active out there. Metal vinyl is, in the grand scheme of things, a very small, fringe market with no mainstream appeal, so there is a relatively small number of people you can sell to. Also, metalheads & 'real' metal collectors are a notoriously poor bunch; we consider items to be getting pricey when they hit 20 euros, which is PEANUTS in many collectibles' markets. Thus, anyone looking to build a retirement portfolio or make a living by wheeling and dealing is not going to invest in Metal Vinyl; they're going to invest in other commodities that have a larger market and are thus easier & more profitable to sell. You can make some money at it, sure, but compared to working in real estate, the stock market, antique furniture, jewelry, etc, the amount of money is paltry.
So, if there are only a handful of 'metal stock brokers' out there, then everyone else who is collecting records must be doing it for some other reason. Some options:
1- they like the music and thus qualify as a metalhead + a collector
2- they don't like the music and are just buying it compulsively (i.e., hoarding stuff without a real need or reason for it). This has to be really rare when it comes to metal records.
3- they don't like the music but want to fit into the subculture and thus are doing it to 'fit in'. I think this is also pretty rare in reality. If you don't enjoy the music, why would you want to fit into a subcullture that revolves around that music? I'm not saying that it never happens- it does sometimes- but I don't think it is as common as some people claim. Are there really thousands and thousands of kids out there bidding up copies of 'Persecution Mania' who only want it so they can try to impress the hot chick in the 'Maiden T-shirt and tight jeans at school?
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Re: do buyers and sellers have to be upfront about a fair price for an item... I've always considered it both parties' responsibility to know what they are doing. If you pay too much or sell an item too cheap, that's your fault for not knowing what you are doing. If I find a member of HAWT CHEEKZ today and he offers to sell me his last 2 copies of their old "Hawt in the Crackz' single for $5 each, am I obligated to tell him they're worth $200 each? IMO no, I am not obligated to do so. The exception would be if I told him flat-out that the records were worth nothing, knowing I could turn around and sell them easily for $200 apiece. Lying is one thing, but buying/selling something for an outrageous price (high or low) doesn't mean you did something dirty IMO.
There's my 2 cents. Have fun.