Some companies do demos, some do resales, some do both, some do neither, some do one or more with transfer fees. In order from best to worst, here are some of my thoughts. I'll just mention some of the big players I've bought from. Again.....just the major players that I've dealt with.
1. VSL Can demo (14 days) and resell....with a moderate fee....expensive libraries....although, I've loved every lib I've bought outright or bought from demo...and I never wanted to resell. They are that good. Lately there have been some very good sales to take the edge of the higher prices. I've spent more money at VSL then any other developer. No regrets.
2. Bestservice. No demo, but resell with moderate fee. Again, never wanted to sell anything I bought from them. I've spent the 4th most money here. No regrets
3. Native Instruments. No demo, but resell with no fee. I don't like NI very much, but for reasons other than what you are inquiring.
4. Cinematic Studio Series. No demo, no resell, but products of consistent immaculate quality and very good pricing. I've spent the 2nd most money here. No regrets
5. Soundiron. No demo, but can resell for moderate to high (NKS libs) fee. Difficult to get more than 30% of initial purchase back if you bought at full price. Great company....top notch customer service. I'm not a fan of their company direction, but that's just me.
6. 8Dio. No demo, no resell, but crazy good sales if your can wait. Flash sale prices will usually dull the pain of a questionable buy, but there is the odd stinker that hurts if you didn't wait until a real good sale....and some still hurt even with a good sale....but those are very rare. Love 8Dio overall. I've spent the 3rd most money here. No major regrets.
7. Strezov Sampling. No demo, no resell, expensive but with decent sales. Very good quality control...great updates (free too) and probably wouldn't want to give back any of their libraries anyway, but I would never pay full price for any of them.
8. Project Sam. No demo, but can resell once. Prices are up there, but great quality. No regrets
9. Audiobro. No demo no resell, but generally pretty good quality with fair pricing. Very good with keeping products updated. It pains me to put Audiobro at 9. because I like them as a company and Audiobro Genesis is the only Flawless library out there., but......at the end of the day.....no regrets.
10. Orchestral Tools. No resell, no demo....expensive libraries. Some libraries are cream of the crop, but there are a few stinkers too...stinkers are rare, but most of their libraries are quite expensive. Sales are rare but there have been some good ones.
11. Spitfire Audio. No resell, no demo....overpriced on most libraries. At least 2 stinkers for every winner. Many libs get abandoned (few or no updates). Quality control gets a C-. Sales are nothing special. The company is in love with themselves. Regrets.
My own personal conclusion from being at VI Control for 2 1/2 years is that there are more 'buyers regrets' from Spitfire Audio here then any other VI company. If you are concerned about jumping in the deep end head first and drowing....then my list is a pretty safe bet.