It strikes me the value of permanent items you find is considerable compared to the xp you need to level up (which are set by the amount of "loose gold" you gain).
It seems any campaign interested in xp-for-gold should consider increasing the ratio of gold to items in any loot, to make it less appealing to sell off items in order to level up faster.
After having thought about this, I have a new (better?) suggestion - increase the xp amounts you need to level up.
I'm talking about the table that currently reads like this:
Level Required
1
2 10
3 30
4 60
5 110
6 190
7 320
8 500
9 750
10 1100
11 1600
12 2300
13 3300
14 4800
15 7100
16 10000
17 15000
18 23000
19 35000
20 55000
That is, in order to level up from 5 to 6 you need to go from 110 to 190 xp, an increase of +80 xp. (Unlike the core rulebook system, you don't ever subtract any xp as you level up)
Now let's say you carry around a level 4 permanent item. Let's say an Animal Staff valued at 90 gp. If you were to sell this at half market price, you could spend 45 gp to gain 45 xp - more than half you need to level up!
Do you think players will be tempted to off-load their items in order to rise through the levels? Well, I can't blame you - at least for the lowest levels, you gain much more survivability by gaining a level than any single pesky thingamagog can ever provide.
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Let's say we simply double the xp requirements. Now, you need to go from 220 to 380 xp, an increase of +160 xp. Selling that staff still nets you 45 xp, but now that's only a quarter of what you need.
Maybe this enough to make the players keep their items. At least it's a start.
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And oh. Of course, you double the loot monsters drop so the speed of levelling remains the same. That is:
Level Gold per monster
1 2
2 4
3 6
4 10
5 16
...
becomes
Level Gold per monster
1 4
2 8
3 12
4 20
5 32
...
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In summary, I believe I have found a much more elegant solution than having to mess around with sell prices, virtual currencies and store credit.
In short, you always get half market price, you always get real solid gold, only each xp is worth half as much, so in practice the result is the same:
You gain more bang for your buck if you spend your gold on items than if you carouse it up on xp.